I say no more then 80k. Gates recommends 60k. Timing belts have a bad habbit of breaking at the wrong time. like when your 80miles from home. If you do only intown driving then I would say drive it till it snaps. But (don't know for sure) some say this will harm the engine if the belt breaks while running and you get bent valves.
Mileage adds to wearout of the belt but also weather extream cold/heat and so on add to the factor of life of the belt.
JD
