Speculating on this in the context of today's economic crisis is most likely a mute point. The Queen is in excellent health, so the event in question is not likely to occur for at least another decade - probably even longer. By then, perhaps Britain will be enjoying a thriving economy once again. Before then will be debates over the cost of the Diamond Jubilee - which is only 3 years away.
Wow - 60 years on the throne; and Her Majesty will be only the second British monarch to ever reach that milestone. For trivia, should Queen Elizabeth II still be reigning on:
11 October 2009, at the age of 83, her reign would surpass that of James VI (57 years, 246 days)
12 May 2011, at the age of 85, her reign would surpass that of George III (59 years, 96 days)
10 September 2015, at the age of 89, she would surpass Queen Victoria as the longest-reigning monarch in British history (63 years, 216 days)
26 May 2024, at the age of 98, she would become the longest-reigning monarch in European history, surpassing the reign of Louis XIV of France (72 years, 3 months, and 18 days).