It depends on the model of your car. For Vauxhall Zafira A, it is every 60000 miles or 6 years while for Zafira B, it is every 100000 miles or 10 years. If it has got the engine Z16XEP, then it is every 100000 miles or 10 years. However this depends on the year of manufacture of the car as well and where mileage and age of car is listed, it should be the one that comes first, that is, whether your mileage reaches the limit first or your car reaches the cambelt replacement age.
To be on the safe side, please check your service book and it will tell you exactly when the cambelt needs replacing for your car.