Inspect it at least every oil change. If no cracks or fraying, ride after checking tightness and alignment. Never over tighten though. A belt can last a long time if properly maintained. Or you ride on gravel roads like I have to. The recommended change for my Indian is 30k and depending on what I see I may or may not get it changed. I'd rather stay with a proven and in good shape old belt than start all over with a new belt and the frequent adjustments and alignments as it stretches. That's just my thing.
Gotten far more Miles out of several Rides than Manual suggests, just to name 2 got 70,000 Miles out of my 09 Ultra Classic and 80,000 out of my Victory Cross Country both which looked near new at that stage .. As long as looks and tracks just fine would not be too concerned but it is something that should be checked often .. Thinking of changing my Dyna Belt although still looks and tracks fine is going on 14 years old even though only has 30,000 Miles, figure that age is a factor as well ..