Wash your belt after EACH training. It carries all the same bacteria that your gi. It isnt in contact with your skin as much, but all the other people you roll with touch it, it rubs against the sweat on the mat etc etc.
There is NO reason whatsoever to NOT wash or clean something.
-No time to dry? Get another pair of whatever, problem solved.
-No washer? Go to one. Or get several pairs of whatever gear so you can make it with only washing your laundry once. You know, like you have several pairs of underwear, so you do not have to walk around in filthy drawers..
-Cant bother? Stay away.
a REALLY easy rule of thumb, sponsored by me:
WASH ALL YOUR TRAINING GEAR AFTER EVERY TRAINING! ALL OF IT!
Seriously. All of it...
Washing a belt with any frequency will make it distort badly.