Timing belt replacement should be done according to the mileage interval in the owner's manual, for instance every 60K. If the belt is exposed, it will wear faster, and oil leaks will soften the belt.
It would help to know the year, make and model of your car. Some engine are interference engines, and can be internally damaged if the belt breaks or strips, others it's not damaging just inconvenient.
Be wary of add-on jobs, they may be trying to get more work out of you while/if they can. Sadly it's not uncommon in this profession.