Depends on what you're doing with the jeans. I've never worn my normal jeans out I'm just a year. I also only wash them if they smell or are visibly dirty. If I'm doing something particularly messy or rough, I use my older jeans. It's similar to the old wisdom that two pairs of boots used together will last longer than two pairs consecutively, because you can "rest" the wet ones which puts less wear on them.

Someone else mentioned Carhart. I have some Duluth cool max pants that are also highly durable.