When to change the top end on a motocross bike?

Motocross bikes are made to race. For the size and weight of the engine, they put out a lot of horse power (HP). Unfortunately, they have a rather short engine life. The most common engine parts that need to be freshened up, is what's refereed to as the top end. This means, the piston, rings, gaskets, and some other smaller parts. The top pro motocross racers, who are fully sponsored by a manufacture, have the fastest engines. But, this means an even shorter engine life. They are known as 10 hour engines. Yes, they need to be rebuilt every 10 hours.

But what about a stock engine? How often do they need to be rebuild? Who sells the best parts, at the best prices?