oil change period?

[TrikerR]

how often should i change the oil in a pretty hi-perf 4 stroke 125cc pit bike. im thinking every 20-30 hours but im unsure. it will be used to race and freestyle and just generall shananigans.

[Yamahauler]

I would change it as soon as it turns a little dark, I dont know how long. Oil is cheaper than motor rebuilds.