Your asking a very broad question.
There are so many variables that could matter in your situation.
As long as your bike is jetted, fuel mixed right, not ridden past motors limit ex.holding wide open long periods of time, uneccesary over-revving, you could get at least a season out of the motor.
In my opinion the stock cast pistons last much longer than the lighter forged wisecos, but the wisecos seem to make alittle bit more power.
My bike is a 2000 and still on stock cast pistons, I have 21 cc coolhead and its makin 145lb psi in each cylinder, and starts just as nice and runs just as smooth as when I purchased the bike. I do not baby the bike, but on the other hand I try not to over work the motor constantly. I would say I ride the motor right to its limit consistently.
I have had it for 3 years now, no troubles
So you tell me how long will your bike last.