Every 2 weeks at best. We basically just use the smell test, unless we look at the towel and think, 'wow, I can't remember ever not using that towel'. Then it's time to go. Once we start our long bike trip, we'll be carrying the little quick dry towels and will hand wash those whenever we have sun to dry them on the panniers.
Jeans? I have two pairs and they will wait at least 2-3 laundry cycles (2-3 weeks) between washes. I change my underwear daily and am not crawling around in the mud, so I just wait until they feel dirty and wash them. It helps that I'm still stuck in the office wearing slacks most days. Once I'm outta there, I'm sure they'll be washed way more frequently.