When should a timing belt be replaced?

to tell you the truth is better to be safe and change your belt around 90,000 miles and while youre at it have youre water pump changed also.I waited to late and just had to buy a new engine.

Depends on the make, model, year, and engine in your vehicle, none of which you list. It ranges from 60,000 to 110,00 miles. Some cars have interference engines and as such if the belt breaks you will have internal engine damage. Open and read your owners manual or call your local dealer for a correct answer.