It really depends on how much and how/where you ride it. I'm on my third top end, and I've only had mine about a year and a half. But I ride almost every day (even in the -10* weather). Some of the symptoms of a worn top end are: hard starting, loss of low end power, loss of top end power, decrease in fuel mileage, low compression. There are a few other things that can contribute to a worn top end. Like the fuel mixture you run, the condition of you air filter, the temp it's run at, the ammount of warm up time the motor is given before being ridden.