A timing belt usually lasts between 40,000 and 100,000 miles or 4-6 years. Each manufacturer recommends a different interval for a replacement, so you should check your vehicle handbook or manufacturer forums to find out yours.
During a car service, a mechanic will also advise you on your timing belt replacement intervals. If it is coming to the end of its lifespan, they will suggest a cambelt change as soon as possible.