IMO, it appears there is no consistency with primary belt issues from the guys I ride with. One has about 800 miles on his first belt. The belt just wouldn't hold rpm anymore. Didn't appear delaminated and no noticable cords/threads being pulled. I finally went through my first belt on my 12 pro. It has close to 700 miles, with about 550 of them boosted, no torque arm or aftermarket motor mounts. Rmscustom stated he is getting 350 to 400 out of a belt with the torque arm and new motor mounts, this is probably the worst I've heard of belt life on the pro.
There are two different belts for the primary belt. I don't recall the numbers for sure and I'm too lazy to go out to my sled and check, but if memory serves me correctly the 115 belt is the best belt and not sure on the number for the lesser belt.