Controversial statement time: If you've worn your gi to train, you should wash it. It boggles my mind when I hear from people who go multiple classes without a wash, even from people who don't sweat much. Are you under the impression that your gi is good for a few classes in between washes? Give it the sniff test right after taking a shower. So instead of asking when to wash your uniform, let's talk about why you should wash it after each and every class!
You should wash it because it's a sponge for bacteria.
The two common gi materials, cotton and polyester, are bad news for different reasons. Cotton is highly absorbent, wicking up bacteria-laden moisture. Since cotton holds onto water for so long, it's an ideal home for bacteria right up until it's washed and dried.
Polyester doesn't absorb so much, but it's actually worse for bacterial contamination. Polyester is made up of tiny, artificial threads extruded via an industrial process, which leaves the surface of each strand covered in hundreds of microscopic fissures and cracks. You can't see these voids... but bacteria can live in them, and once they've dug in, even a vigorous wash won't get them out. It's much easier to prevent these bugs from setting up shop by washing frequently than it is to try and flush them out after the fact.
You may wonder why this is a big deal: after all, there are bacteria everywhere. Unfortunately, the environment of a martial arts school is perfect for the spread of harmful bacteria, from funk-causing Corynebacterium to the ghoulish, flesh-eating MSRA. It's even possible to pick up gym diseases like ringworm if you don't keep your body and your gear clean.
It's much easier to prevent a stinky gi than it is to fix it.
Once bacteria take up shop inside your gi, it's very difficult to resolve – and a gi isn't cheap, so you won't want to replace it unless it's been totally destroyed. My heavyweight cotton gi was afflicted by the dreaded stank once, and it took several weeks of hot washes, bleaching, and airing-out time in the sunlight to get it back to normal.
When to wash your obi
The obi, or belt, is subject to many traditions. At many dojo (including mine!) the tradition is not to wash your obi under any circumstances.
I hate to admit it, but this isn't a rule I follow (sorry, sensei.) I'm even going to advise you to break the rules, for the sake of your uniform and your training partners! Here are the times when I strongly suggest you wash your belt (and don't forget, if you have a cotton belt, don't wash it on hot unless you're OK with some shrinkage!):
Your obi is stained
At one point in jiu-jitsu class I cracked a nail and bled everywhere, including on my belt. My wife washed the belt; at the time I was dismayed, because my bloody belt looked really cool. But it was for the best.
Your gi has an odor problem
If your gi is showing the signs of stank, the same bacteria are on your belt. All the bleach and sun exposure in the world won't help your gi if you're tying a dirty belt around it every time it gets clean.
Anytime you notice your uniform smells bad, wash your belt. And generally, if you can smell it, wash it. It's just common sense.
When it's been six months since the last time it was washed
If it smells fine and isn't dirty, it's tempting to throw it back in your gym bag. But as we've already learned, once your uniform starts to stink, it's way harder to make it fresh again. Bacteria inside the fabric are hard to get rid of, so don't give them a chance to reach critical mass.
If at all possible, air-dry your uniform on a clothesline in the sun. If this isn't possible, air-drying in a room with a dehumidifier is a great way to suck the moisture out overnight, without causing undue shrinkage.
If neither of these things is feasible, you may want to consider buying a uniform one half-size large, and shrinking it down in the dryer (or purchasing a poly-blend uniform, I guess – these don't shrink in the dryer much.) One of my uniforms is a full size up, and while it's still a bit baggier than I'd like, I can wash it on hot and dry that sucker on blast every time without worry. If I mess up and don't have a gi ready ahead of time, I can turn that one around in under two hours.
There are a few situations where you need to take extraordinary action.
Your gi still stinks after a wash
This is pretty much a worst-case scenario. You get home, throw the smelly gi in the wash, throw it in the dryer, and... it still reeks. It might even smell fresh out of the dryer, but stink when you put it on at the dojo. I've had this happen, and it's awful.
Step 1: Take action.
The thing to understand here is that a bacterial colony is occupying your gi. Just washing it isn't going to do the trick anymore: the best solutions are antibacterial detergents, bleach, soaking in very hot water, and drying it in the sun for long periods of time. This can be a real problem if your uniform isn't already shrunk, or if it's a color other than white: in these cases, your best bet is to hang-dry the uniform in the sun for days at a time.
Step 2: Be patient.
Once you have a serious stank situation, it's probably not going to go away as quick as you like. Sun-drying works, reportedly, because the ultraviolet light is harmful to bacteria. What it doesn't do, though, is work fast. I've had good success, though it took five wash-dry cycles before the gi returned to normal.
Step 3: Have a backup.
If at all possible, take your stank-gi out of rotation while this is all going on. Nobody wants to smell it, and working out in it is letting those bacteria build up again.
The solution? Wear another gi. If you don't have a backup, this is a great excuse to buy one. You could experiment with a different manufacturer, or switch to canvas, or go up a half-size to let you dry it on hot.
The side ties rip out
This is my least favorite uniform issue, because repairs are difficult and don't tend to last as long as the original. First, though, you'll need to get some hardcore thread: I use upholsterer's thread, which is much thicker and more resilient than normal thread.
Next, you'll need a tie to stitch onto the gi top. Usually the old tie breaks close to the stitching, and there's enough length to just sew the old tie right back on. If that's not the case, you can purchase a set of replacement ties quite affordably from Mei Jin.
Finally, you'll need to sew it in place. Usually the original tie stub is actually sewn under the lapel's runner stitch, so removing it would be quite destructive. For this reason, I sew the new tie directly over the other one, stitching a box-shape into the tie to ensure it is well supported.
The waist tie binds up and can't be tightened
If you have an elastic-waisted uniform, this won't happen to you! Many heavyweight gi have a canvas strip that is wrapped one and a half times around the waist, forming an extra-strong drawstring. Sometimes, these can become difficult or impossible to clinch shut, which results in the dreaded pants-falling-down scenario. There are two sources for this problem:
Reason 1: The ends have twisted
Carelessly pulling the ends tight can cause the leading end to wrap around the second loop of the drawstring, which increases friction. You may also be twisting the drawstring when you pull, causing the entire cord to twist tighter a little bit every time you dress for class. The result of either is a drawstring that can't be closed!
To solve this, carefully rotate the entire drawstring by bunching the gi on one side, then pulling it free on the other. Repeat until you have exposed the covered part, and carefully detangle the cords.
Reason 2: Your drawstring has frayed
Tying your drawstring causes the cord inside to rub. The folded canvas drawstrings can shed, releasing long fibers that tangle tightly around the drawstring.
To fix this, expose the inner section of the drawstring as described in the previous example, and carefully pick off the matted fibers.
Your jacket or pants rip
This is an unfortunate situation, because generally when your gi rips, it's somewhere that takes a lot of stress, like the shoulder seams or gusset. In these cases, making a proper repair is exceptionally hard because these seams are generally triple-stitched or better, and after a rip there isn't usually enough fabric to re-sew the piece with any strength. A single-stitched repair probably won't even last a single class!
Generally, unless you're really lucky, a rip means you have to live with a shredded uniform or replace it. But that doesn't mean you have to throw it out: wash it, dry it, and store it carefully, and you have valuable matching fabric for patching your next gi or making replacement side-ties.