Contrary to a blog quoted in yesterday's Guardian, it is November 22, 2008 (not 2009 as stated) when Blair would break Thatcher's record. Sadly for Blair this time of departure would probably leave too little time for an incoming leader to prepare for a 2009 election, so Labour are unlikely to want him to go on that long.
A more likely date of departure would be to have the new leader in place by September 2008 in time for the party conference. This would leave Blair 2 months short of Thatcher's record, which would be tough on him but the party and country must come first. And this country needs the Labour party more than Blair breaking Thatcher's record, much as Blair deserves the accolade for taking so much abuse from the right wing (and left leaning) press and still winning 3 elections for Labour.
By the way Mr Blair, if you really want to overshadow your Iraq legacy, then do us all a favour and hold a referendum on PR, like you have promised in all the manifestos.