I think 10 years is a good goal depending on your hours, but if you are running 1,000 hours a year 6 years might be a good goal also. Indyallpro from here flips his every 2-3 years. 2-3 years is probably when you are going to get the most back from you investment as far as resale, but with higher priced used equipment I believe it will be harder to sell(maybe not?).
For me it comes down to running the hell out of them till they throw a rod or something because it's still making me great money for something that is already paid off. My Peak has around 4,500 hours and it just won't die(might just throw another motor on it when it does, van runs great(looks OK) with only 93,000)! It does have some to do with mechanical ability also, if your not able to fix things yourself I would stick to the 5-6 year thing, or 5,000 hours.
I think I might have been able to get $8,000 for my back up unit,and van in the beginning of last year, well this year it made around $20,000. So instead of having a new payment....