Recent experience....Fan belt broke after 1700 hours of use and almost exactly four years of time. It was a Gates Hi-Power, good belt, installed in fall of 08 when the genset was upgraded from SpecA to SpecB. The design of airflow is such that all radiator heat, as well as all engine heat flows over this belt before exiting the genset, i.e. runs very warm it's whole life.
FYI...if you have a SpecA you have two belts instead of one, a plastic idler pulley, and a plastic cooling fan. Mine was upgraded to SpecB while under warranty by Cummins Power South in Tampa, as the plastic parts started to break at 860 hours.
SpecA to SpecB if you buy the parts is almost $200, but you end up with an aluminum cooling fan and a hub adapter that changes you to a one belt system. The belt number is 511-0218 from Cummins, who will not cross it for you. It turns out to be a NAPA A38 which is a 40" x 1/2 belt.
Just changing the belt on the SpecB is about three hours of old guy time, probably 3 or 4 flat rate at the shop. You could change the A to B with a little extra work if you are willing to remove more sheet metal. The B belt change only requres removing the top and side/end pieces of the cabinet, unbolting the cooling fan so you can sneak the belt through, then buttoning up.
Fortunately this broke just as we were arriving at Gulf Islands NS in Pensacola, where we had a week of down time, with hookups, so it was not a big deal. If it had broken the week before running through Texas in 108 degree weather....it would have been the pits. :(
Hope this helps someone out...and if you are getting up in hours, get a spare belt, or the upgrade kit. Murphy is not always as kind as he was last week. Oh yeah...and of course we had to make the trip over to Maguire's, I think Murphy was Irish and broke the belt to send us back to Maguires.