Technically, a cambelt should be good for 40k, but i think its wise to change it once every 2 years or 25k, withever comes first.
Its also wise to get vauxhall to do the job, simply because if it breaks within its lifetime causing no end of damage, they will rebuild your engine for free. This isnt a widely known fact, they wont advertise it, but its very true, and good insurance.
Just keep the receipt lol.
Not cheap having it done by a dealer i admit, but peace of mind.