I'm generally pretty clumsy and get some kind of food on mine. So, one to two wears. I agree with the commenters who said that washing frequently keeps them looking crisp and clean. I also agree that they wash up nicely. I have a stain stick from Ecover, and when I take my jeans off for the day, if I see any specks of crud, I moisten the spots and treat them with the stain stick, and they come right out.
If you want to try white jeans, find yourself a cheaper pair and see how they work for you. That way, if you find it a pain, you're not out much. American Eagle has very reasonably priced white jeans, though I think they are jeggings, so they will be superskinny. But they are definitely jeans and not leggings. Loft and Ann Taylor also have been running some end-of-season sales where you can get a good deal.