Officially from the manual and from the lack of any information from VM (who don't share much as it is) there is only a mileage spec on the timing belt of 100k miles.
HOWEVER:
I have now worked on over 30 of these. I have seen belts that were over 100k miles and some that were under. All were the same rough ages because of when they were installed, with the exception of 3 that the engines had far in excess of 100k miles and these were belts #2 becoming #3 for each.
I have not found ANY direct environmental causes to say "this has caused this belt to prematurely age" such as general temp or humidity (or lack of). Miami Florida is hot and humid, and the belt at 8 years and 80k looked terrible. Austin Texas is hot and mostly-dry, and at 85k miles and NOT 8 years old, two belts owned by the same person looked terrible. Big cracks. Another in Austin a few miles away looked mostly-fine however. Go figure.
Boston, Mass is cold and wet, and at 90k miles and 7 years, the belt had cracking. Phoenix, Az is hot and VERY dry, and at 90k miles and the belt looked decent. LA is mild and dry (until it isn't either) and the two belts there were in good shape. Albuquerque is cool and very dry (and very high up!) and the belt looked decent.
The point? You want your belt to be in the "decent" category, which means that the rubber doesn't appear massively dried and cracked, chunks aren't missing, and the cords aren't exposed because of huge cracks. I have been placing the stickers on each CRD that I have worked on, with a 100k mile and FIVE YEAR timeframe on the belt, based on my observations of the conditions of each when they came out. In a variety of climates and operating conditions, to safely say that each belt will 100% last x number of years is very tough - So I am choosing to err on the side of a wide safety margin. I think that Chrysler / VM failed us (gee, that's unusual) in not providing a time spec for the belts, as Gates / Goodyear ABSOLUTELY would have given THEM one. I do not know what the manufacturer's number is however, because I get the amorphous and weasel-word answer of "It can vary by application, ask your vehicle maker" as a reply.
Generally speaking however, rubber belts are not specified to be in service much longer than 5 years, and for something as critical as the timing belt, if it hasn't been changed by now - it NEEDS to be changed.
My main gig, working in television, gives me a flexible schedule to be able to help out the group here. As long as there isn't (too much) snow on the ground or severe weather in the sky, I'm game to go just about anywhere and do these timing belts for the members here. The more people that sign on for a given route, the better the spread of the travel costs for everyone. I haven't run the final numbers for the last trip out to LA and all the CRDs that I worked on during that month away... But I suspect that you guys all got a fantastic deal, certainly compared against the uncertainty of bringing it to a dealer or "cummins / powerstroke" mechanic that has never seen inside this motor before.
So would I be interested in possibly going to the UP of Michigan? "Yah, shore, you betcha"
