well, that's a hard one to vote for. simple answer is when the rings/cylinders are worn. every engine is different. if you have reason to suspect that it's time, ie. poor low rpm operation, abusive life like cheap oil, air leaks, dry starts, contaminants, etc. open her up and check. you can potentially go over 100 hrs on a good motor, or you can neglect one for 20-30. i have a blaster and king quad baught in 94, and the blaster is still running strong, and the suzuki needs rings.