Belts wear due to use or age. If you drive a lot (25k a year) you may be able to get well over 60k on your belts. If you don't drive much they will begin to dry rot/crack eventually.
If I don't see any fraying/cracking I keep on truckin.
I don't normally replace them until one starts squealing, then I replace them all at once. If you have any of the above symptoms, it's important to replace the belts soon. If your PS belt broke while in the middle of a curve, it could cause you to have an accident.
Timewise, 8 years or so seems to be the magic number, no matter what the mileage.