I had a friend who never touched his yz125 for 7 years, then I've heard that people rebuild every month if they are heavy into racing. For the average rider 60 hours is a good time to check the top end.
Depending on how big you are and your skill, it could go either way on size. If you are young, around 115-140lbs a 125 is going to be a good idea. Now that doesnt limit you to only the 125. The 250 is twice the bike and only a few pounds heavier, but you would need to be able to ride it with some control. But if you plan on keeping this bike for a while and you have previous riding experience, I would go with the 250. Same thing being said if you are older/heavier.