I've been out of snowmobiling for a while, but one of the best things to check for belt longevity is the "center to center" distance from Primary clutch to Secondary clutch.

 Hopefully, there is some adjustment to move the engine forward or back, to alter that center to center distance.

Along with center to center distance, there is clutch offset, that is adjusted at same time (they make tools to help do this and keep thing square)

Getting this adjusted correctly, helps that "belt transmission" start out in "1st gear" when you accelerate = much easier on belt.

Note: Also worn belts, are like starting out in 2nd or 3rd gear; so sometimes all that is necessary, is to install a brand new correct length belt, and you are back to starting in "1st gear" again.