It all depends on how you ride it, what oils you use, and overall how well you take care of it. 

If you ride weekly or so, best thing is plan to replace the piston or just rings each year with maybe a hone.  If a hone won't clean up the cylinder, you'll need to bore it.

If you have excessive wear after a year make sure your jetting is right, make sure you're using a quality oil at an acceptable ratio.  Maybe you ride hard and enough that it will require a piston and rings every 6 months?  It's all up to you really.

Don't wait until it explodes or you have low compression.