A lot of people say with great conviction that you should never wash jeans and will refer you to the CEO of Levi's who's famous for this claim, but his main reason for that is water conservation and has little to do with hygiene or durability. Water conservation is a fair reason not to wash them every week for the most part, I think, but never is quite a long time. I say trust your nose. I'll wash mine once they start smelling like they need a wash, basically. That tends to be every 2-3 months or so for me, at an estimate.