At the very least, change it when it needs it - if you notice checking, missing ribs, frayed edges, or excessive squealing - time to change it. Cracks running perpendicular to the longitidinal ribs are normal. Some owners get as little as 25K out of belt - some can go 100K or more on the same belt - depends on driving conditions, age, amout of ozone belt is subjected to, etc.
Belts are pretty cheap - if in doubt, change it and keep the old one in the trunk as a spare. Simple, under 30-minute job perfect for a DIY job with basic tools.