For the serpentine (accessory) belt, 60 - 70k miles is a good time (or sooner, if it needs it). YOu can tell by looking at the ribbed (drive) side and if there are chunks missing - replace it. If there are just small cracks, but no missing rib sections and the back side is not showing fibers, you are still OK.

Timing belts are a lot trickier, and this is like gambling. You can go the 105k miles like my manual says ('01 3.2 CL), but I will change at 65k miles or so just to insure I don't end up needing new heads and pistons.

The harder you drive it, the sooner you need to change the belts. Also if you have a problem with one of the accessories, then that can damage the belt. The serpentine ones are cheap, but labor to install is significant, or a DIY is a b*tch.