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For the Conservatives, Margaret Thatcher has been the longest serving post war PM, at eleven and a half years, whilst for Labour Tony Blair managed nearly ten and a half.
What is the longest both could realistically serve for before falling either to general election defeat or own party machinations?
Never one to not put some ideas out there - I would've thought perhaps 1992 for Thatcher, if you can butterfly away (or have Thatcher beat) Heseltine's challenge and then fall to defeat in 1992 to Labour.
For Blair, if he can get rid of Brown sometime in the 2001-2005 parliament and then win again in this Alt-2005 he could (I suspect) manage to struggle on to 2010 before losing to David Cameron.