A little theory of mine:

1. Margaret Thatcher won the election of 4th May 1979 (JD 2443998)
2. Margaret Thatcher resigned on 22nd November 1990 (JD 2448218)
3. Margaret Thatcher was in power for 4220 days.

4. Tony Blair won the election of 2nd May 1997 (JD 2450571)
5. Hypothesis: Tony Blair wants to beat Thatcher’s record.
6. ergo: Tony Blair will do his utmost not to resign before JD 2450571 + 4221 = JD 2454792
7. = 21st November 2008

Julian Date conversion via [aa.usno.navy.mil]