When to replace timing belt

I just bought my first Chevy, a 2003 S10 LS Pickup with a manual tranny, 4 cylindar engine and 85K miles. The truck is exceptionally clean, and the engine compartment immaculate. She drives like new. My question is when should the timing belt be changed? Unfortunately, I don't have any maintenance records for the truck and the owner's manual doesn't even mention a timing belt.

Thanks.

s10s have a timing chain :)

Thanks for the correction. And when does one replace said chain?

there isnt a sceduled time to replace them most say just wait untill it breaks or whenever the engine dies.

I take it that unlike the old days, if the chain breaks it doesn't destroy the engine? I was always under the impression that the chain should be changed at regular intervals, albiet, not very often but certainly before it actually breaks.

Thanks for the info.

Timing chains will stretch over time, which will cause problems with valve timing, but they seldom ever break.