Although could be held at any time between now and then.
The decision to call a General Election rests with the Prime Minister.
Accepted practice is to plan to have one four years into a term (to give you one year to spare in case something goes wrong). This is why we had elections after 1997 in 2001 and 2005.
As a result its not unreasonable to suggest it be Summer 2009, although the powers that be may want to not coincide them with that years European elections, which cannot be moved.