I usually wash a towel a week, as long as it's not bunchy on the bar so it can completely dry out.

every week (barring guests or other extenuating circumstances), just like my sheets. kitchen towels get changed out more frequently, pretty much as soon as they seem kind of iffy for touching food/eating surfaces.

I second Cheryl K. Replacing it once a week is pretty standard, sometimes more frequently. Either way, it certainly has to be allowed to dry out completely!

I'd say every 7-10 days.

weeklyish, depends on what elese i'm washing and where it fits into the wash schedule

It varies, but I'd say something between 3-4 weeks. Is that bad? How often are you supposed to change them?

Weekly /-

They start smelling stale after a week, thats usually my cue

Monthly or less when the weather is warm (so the towels dry faster.) Right now I'm thinking I won't get away with that much longer.

Generally, weekly. Variables like time of year and how often I'm showering a day* can make this occur on a shorter time frame though.
*before work and after the gym/hockey/dance class

I change them after they where used once, I don´t know why, but I´m used to it and allthough you are clean after showering or having a bath, it is used and used = dirty for me.

It's an every use occurrence for me. Used to be that I was anal and just thought that a towel was disgusting after it was used. Now...well...now I'm disgusting and can never seem to remember to hang the towel after I use it. Instead I throw it on the floor and the cats end up using it as a bed.

Once every other week, along with my sheets. Used to be monthly, but i've stepped it up a bit.

Honestly cannot understand the folks that consider a towel "dirty" after one use. All that's saying is that your newly clean body is too dirty, because its coming off on your towels. Since you're never cleaner than you are when you've stepped out of a shower, why would it make you feel dirty to use a towel that's got yesterday's shower on it? To me, the only reason to wash towels at all is to remove whatever dust may have accumulated over a month or so. Or if you haven't let it dry completely and its got some smell going on.

Probably every 3-4 weeks as long as it doesn't smell. Unless it's a guest, then they get a clean towel and it's washed as soon as they leave.

Usually I use a smaller towel for my small bits and feet, so I don't have to use a new one every time.

Correcting; The small one I only use once but the larger can last almost a week depend if I forget to use a small one then I'll have to wash it.

I change towels once a week.