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Some of the basics would be to lube the chain at the start of every ride and if you've ridden for an extended period of time lube it again after you take a break especially if you are riding in harsh conditions.
We changed the oil every ride on our two strokes, man leave it much longer, you'll have to decide what works for you but I'd likely not leave it past 5 hours of use.
The air filter is the only thing keeping dirt out of the engine, again we change it every ride, many others only seem to do it when it's filthy. Sucking dirt into the intake is bad and gets very expensive so you have to decide on an interval that works for you.
Top ends on a 125 we usually started the season fresh and then did it again halfway through the season.