If you don't know the cars history, then I'd advise 40k miles and every multiple of that.
The official figure varies from this to 60k miles (80k has even been suggested before now!), but for peace of mind, I'd get it done early.
You are also best to spend an extra £25 and get the water pump changed at the same time, as there is no extra labour involved and it'll save you a fortune if it goes!
Cheers,.
Mike