It is 40000 miles for the change, but I don't remember the time interval.
When my Astra had done 36000 I took it in to a garage under warranty for a waterpump renewal, the garage asked if I wanted to have a new cambelt fitted as a precaution so I said yes. Only cost �40 as they had to take the cambelt off to do the water pump.
30000 miles later the cambelt snapped, I had moved 250 miles away from the garage then but I am convinced the cambelt had never been changed at the garage.
If the garage who did the cambelt on yours have not fitted a sticker with dates of the cambelt change you have got to assume it has not been done.
If not then the origonal cambelt only lasted 66000 miles on mine so get yours done now!!!!