Normally it's every 2 years or 12,000 miles - whichever comes first.
Even if it's just sat in the garage the belts go hard and may start to crack, and could eventually break. They also harden around the cam gears if the engine isn't rotated often and the belts stay in the same position, so can end up in rather funny shapes which could jump the gear and offset the timing and you'll end up with a piston-valve interface issue!
If you've not had it done in the last two years it's probably worth having done - and the rest of the service to check what else needs to be done before it causes you real problems!
Where abouts in the country are you? Maybe we can recommend someone...
I'd rather die peacefully in my sleep like my Grandfather, than screaming in terror like his passengers.- Jim Harkins