The serpentine belt you can just inspect. look at the inside ribs for cracks..if you see a lot in an inch, time to change it. If no cracks, then it is still good. I dont think there is any set timeframe for this. I have almost 60,000 on my truck and the belt still looks good.
2016 GMC Sierra 2500HD Denali
Extang Blackmax Tonneau
Weathertech floor mats and mud flaps