2 strokes do, generally, require more attention. Also, generally speaking, the smaller the 2 stroke, the more the attention (or frequency of rebuilds, etc). The 85's and 125's are run pretty wide open much of the time, so they require top end work sooner. They should be called GTO's (Gas..er premix, Tires and Oil).
Your XR is gas and go. The CR/YZ/RM/KX you have to watch the premix (keep it fresh), drain the carb if it sets for long periods, clean or change plugs every few rides, and keep the filter clean.
I would suggest Eric Gorr's excellent book "Motocross & Off-Road Performance Handbook" even if you don't go the 2 stroke route. It pretty much outlines everything. It states a tear down as follows:
80cc 5 hours/125cc 10 hours/250cc 20 hours/500cc 40 hours
PS. I don't have the service manual for th CR5 yet, but my old '84 CR250 stated you should replace the piston after every 7 hours of racing. That's real extreme for general riding, but maybe not for those who want/need to have the best.