I believe about 4 years. Latest time for the government to call a General Election is May next year. I'm almost certain about that, but length of office I'm not so sure about. I'm only 14, so a few years ago I wasn't exactly politically aware.
Ah ok... I just remember a project back in like 10th grade (I think) which was back in 97-98 and Blair was in office. same as Chirac from France so it struck me as a while