Working in a salvage yard, i saw that most cars blow the belt if not changed at around 50k over the reccomended interval. Driving styleand other factors can affect that however. Definitely check for signs of dryness and cracking! That doesnt mean your not in need of a replacement if it looks fine. Ive seen decent looking belts snap too. I would change based on this equation:
New timing belt=2hours work$45.00 in parts OR
Snaped belt= $400.00 in used motor and associated down time and unforseen headaches.

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04' Olds Alero
91' Colt GL hatchback
4g15 12valve 5spd.
4g37 TB, MAF & Injectors. 4g37 ECU
Ported & polished & custom CAI w/K&N filter
5* Cam retard, 7*Ignition advance
2-1/4" catback w/flowmaster muffler
89 GT brake and suspension swap
Lots of little s#@t