Who often do i change the oil on a kx250 - 2005 kawasaki KX 250 F

If you race this bike then I would suggest at the end of every full race meet dump the oil and filter as apart of the clean down to put away for next race day routine obviously fill it back up when due to race next
If your just recreational use and not going on the rev limiter all day long then about every 2 - 4 hours of running time would be OK
simple rule of thumb with the four stroke Motocross bikes if you love the bike and you can afford it change the oil as much as possible you cannot change to often in my opinion (just don't over tighten the sump plug just nipped up is fine)
cheers mate happy riding