I think I remember reading some shock industry material that suggested replacing parts in the 25-40K mile range. I've had some Gabriel Terrain-Tamer shocks last well over 60K miles and have plenty of pressure and damping left in them. I've also had OEM replacement (read: Mopar from the dealer) struts last MAYBE 10K miles before they were outright flacid. Same can be said for some Bilstein HD struts that replaced the Mopar units... maybe 20K out of them tops. It's not just Mopar parts that suffer, either. I have a set of OEM shocks/struts from a relatively low-mileage 2008 Honda Fit, and the damping is inconsistent between the pairs already. KYB supplies thes shocks/struts.
In the end, you may get lucky and get a bunch of miles out of a set, but you really have to keep your eye on them and stay tuned in to how they're working. Unless they die a tragic death, you may just get used to the gradual decrease in performance and one day you realize you're driving a wallowing pig...
Clair