I have a set of 32" Pro Comp MT tires that I used primarily for off-road. That have about 14,000 miles on them and they still have a lot of tread. I think they should get be good to 40,000 miles before the tread is worn enough to not be useful off-road.
I have noticed they are beginning to show signs of being out of balance. I had them balanced at a shop when I was traveling but it didn't help a lot. It may be that they did not do a good job. I'll have it done by a place I trust to see if it improves. Balance is not a big deal off-road but I like it to be smooth on the highways while driving to an off-road trail.
2002 FX4 Bright Red Supercab 4.0 SOHC, manual 5-spd & Tcase, 33" BFG MTs, K&N air filter, Modified air box inlet, Gibson SS cat. back exhaust, Bama flash, Hidden Hitch round receiver, Rear helper air springs, Electric trailer brake controller, Line-X bed liner, WAAG bar with KC lights and Wildernest shell.