What year model and which engine you got??? Any mention of time/mileage in the owners/operators manual?

My guess is, change at least every 60,000 to 100,000 miles, maybe sooner if it's cracking and loosing chunks.....

If your serpentine belt is cracked or has chunks missing, it needs replacement.

Also change out the tensioner and idler pulleys, you'll be glad you did.

I change mine every 2 years, and my yearly mileage is only about 15-k per year on my haul truck.  I change my because of the heat down here will kill it just as fast as miles will.  But actual requirements I would have to DITTO retento on the 60 to 80-k range.  And for 20 or 30 bucks that is cheap insurance.