It depends on the chassis number, but to be on the safe side every 40,000 miles or 4 years. But see the following post.
http://www.vauxhallownersnetwork.co.uk/index.php?threads/astra-club-1-6-2003-timing-belt-changing-frequency.191750/
Don't forget the water pump and thermostat too if the car has done 80,000 miles or more. It is false economy not to do them at the same time.
ChicksFan, August 2013
