Enoch Powell was wrong to say that all political careers end in failure. But for a leader to leave politics with his or her reputation relatively intact comes down to careful timing. When is the right moment to jump?

David Cameron’s announcement this week that he won’t seek a third term as prime minister has sparked a debate in the UK about setting limits on politicians’ terms of office.

Critics have said it will create problems for the Conservative Party leader's second term if he is re-elected prime minister in May's general election; his defenders have said he was right to publicly put a time limit on his premiership. The issue has divided opinion evenly: 38 per cent of Britons back his decision, according to a Guardian/ICM survey, and 38 per cent believe he was wrong.

Prominent figures in the British Labour Party, including Tony Blair’s media strategist Alastair Campbell, have portrayed it as a blunder. This could be because when Blair said, in 2004, that if he were elected for a third term as prime minister he would not serve it in full, he lost authority.

Blair ceded ground to his rival Gordon Brown, and from the moment of his re-election for that third term he was dogged by questions about when he would step down. His cause was not helped by constant plotting and undermining of his leadership by Brown and his allies.

Ambitious leaders

There appear to be no such plots within the Tories. Cameron’s relationship with his chancellor of the exchequer,

George Osborne

, is healthy. Other ambitious leaders are waiting in the wings, including the exotic figure of

Boris Johnson

, but none shows any sign of moving against the Conservative Party leader.

Some say Cameron is wrong to assume the Tories will win the election, but at this stage it does not look like a particular problem for him. What is more likely is that the clock will start ticking once the election is over. If the Conservatives are re-elected to office, there will be an increased focus on when the prime minister will step down. And that is when it might start becoming a problem.

Realpolitik dictates that Cameron would be unable to serve a full second term. His potential successors would be circling, impatient for their turn, and would want a good tilt at a general election. It would mean Cameron would have to step down early.

And if the present calm doesn’t hold he might be forced out earlier than he thinks.

Lame ducks

There is a persuasive theory that the moment political leaders set time limits they become lame ducks. When John McCain stood against Barack Obama, in 2008, there were concerns about his age. (He was 72 at the time.) Some advisers urged him to declare that he would serve only a single term if elected, but he resisted, as it would have made him a lame-duck president, a stop-gap leader with little hope of effecting change.

No US president is allowed to serve more than two terms. Many two-term presidents have been more effective in their second terms, because they can implement policy without having to think about re-election.

Ireland has no term limits for serving leaders – although Renua Ireland does want such conditions. But, like Blair and others, Irish political leaders have been put under pressure to declare their time of parting.

Bertie Ahern’s trick was to say that he would not remain as taoiseach past the age of 60, which would have been in 2011. In the event he did not last that long, being forced out in the spring of 2008 after disclosures about his finances.

Michael D Higgins’s age came up during the 2011 presidential election. He was 70 at the time and will be 77 by the time his term ends, in 2018. He let it be known that he was probably not going to seek a second term.

Minister for Health Leo Varadkar told Miriam O’Callaghan, on RTÉ radio, that he will retire from politics at the age of 50 (still more than a decade away). He might come to regret such a bold statement.

Éamon de Valera's career spanned two-thirds of a century. Charles J Haughey, who was elected taoiseach four times, once remarked, "Some Chinese leaders go on into their 80s."

But most political leaders don’t set time limits, staying on until the electorate – or their colleagues – give them the boot.