I have worn out several pair over a 40 year period. How long they last depends on how good they are (Filstrom (sp) last forever) and how fat your legs are. I am heavy with big thighs and tend to wear everything out, except Cabelas nailhead cotton pants. All of my wool pants have worn out between the legs and in the seat. I was a scout master for 30 years and also went backpacking, hunting and fishing a lot. Most of my wool pants lasted 5+ years at +/- 40 days per year. I switched to the nailhead pants when I could no longer buy whipcord wool pants in my size (52" waist). Filstrom also raised their pants to abour $200 a pair. I think a thinner person could get a lifetime of use out of a pair though.
Ron Athay