Two I reckon. Labour will need the first term to clear all the dead wood out of their party (yes you Harriet Harman) after which they'll realise that there's nobody left and they need another term out of government to dredge up some half-credible new faces. Milliband will probably get a term as Labour leader before they boot him out.
Purnell would probably fancy himself for a crack at the leadership but we needn't worry about that as I will personally assasinate him if he gets anywhere near running this country
