I wash my jeans when they need it. To be honest, you should avoid washing them because it actually ruins the fibres quicker and good jeans are expensive. I've often got a pair of cheap jeans (say hello Primark's best) which I'll wear more often and have a wash every 3 - 4 weeks or so unless marked by spilt food etc.

If I have good jeans like a decent pair of Wrangler's or other brand name, then they'll rarely see the machine. Literally will get washed if they seem to have a stain or something really made them smell. If I feel that they need more than a good airing/freezing then I may drop them off at the dry cleaners.