If you have easy access to a dryer, and line dry just for the efficiency/ cost reasons, you can always line dry, then toss into the dryer with a wet washcloth for like 5 minutes once they're dry. It sort of steams/refreshes them, and they get rid of most of the stiffness. I assume that's also easier on them than a full dryer cycle.
I don't know what the "common" wear-out points for jeans are, but mine last about a year or two of daily-ish wear before the fabric just below the crotch starts wearing out (where my thighs touch each other, presumably). But, I also walk more than the average person, (public transit commute) so that may be part of it.
More expensive jeans, in my experience, have thinner, more fashionable denim and actually wear out faster, so there's a pretty hard upper bound on what I'll spend on jeans.