when adjusted properly, at recommended intervals you should have a better idea when to change your belts. age, stretch, and visual inspection should be enough information to indicate when a t-belt should be changed. BUT like many others I think I'd rather change and throw away a belt that has some life left. rather than wait till the belt fails and replace it and some valves, possibly more. Belts are rubber alot like tires, so with this in mind, would you ride on 2 year old tires? how about 3 year old tires?
personally I draw the line at 3, but others might go longer. belts are subject to slightly differnt conditions, as they run inside covers and dont get the hazards of sharp objects and dirt. but they are subject to heat and cold. and just the wear of running.
In my opinion this general maintenance should fall into the same category as tires, brakes, and oil.
after all, part of being safe is being prepared right?
be safe