Almost 24,000 miles on my car when I stored it for this year and still stock belts (my car is not pullied though) A few open track days and plenty of drag/street use.
I got about 3000 miles from my Gates belts. When they got glazed and started chirping I replaced them. But as far as actually wearing out, you should get 12,000+ with an aftermarket pulley providing it (the pulley) was installed properly.
50,000+ miles on my car....not pullied....a couple open track days and highway straight runs with cars about 2 times per week and the stock belts look as new as the day the car was bought.