I have used belts for over 5000 miles without being able to see any significant wear. OTOH, I have also seen 'belts blow up' which were virtually brand new. I belt taken care of at all times can last a long time but, then again, a belt can be so damaged in a few seconds that it will fail with very few miles on it. Consider everything you've done with that belt. If you can honestly say that you went easy on it when it was new, you've never forced it to turn a cold, stiff track or raced with it before it was warm, and you've never made a bit of 'blue smoke' with it, then you probably have a right to a new belt under warranty.
I've never tried to get a belt replaced because I have probably been at fault in the failure of any belt as defined by the warranty. Some belts seem to be more forgiving of abuse. I haven't seen anything to predict it but I have had the best luck with belts that match the brand of the machine, maybe because I am a bit more careful with things I pay more for.