hi vauxhall say 60,000 and hayns say 40,000 or every 6 years garage price aprox 120 140 but i advise to get a full kit and water pump aswell as these are prone for going and the cam belt runs the pump so kill two birds with one stone hope this helps
I believe it is 50,000 miles or 8 years but for the sake of £75 for a full kit including the followers and tensioner it is worth doing. It took me about 3 hours, as a novice, to do everything and now my mind is at rest for the next 50,000 or so. The Haynes manual is really easy to follow.
Have a look at www.mycambelt.com use the search option to get to the Zafi range and you will see the official advice. Dont forget to find out about intervals on water pump replacement as that is linked to to the cam pully and if that pump fails it will kill the engine.