Prime ministers tend to last longer than you think. I haven’t placed a bet of more than 50p on politics since I lost an undisclosed sum on John Major to cease being prime minister in 1994. It seemed obvious to me that the Conservative Party would replace him with Michael Heseltine or Kenneth Clarke.

Then John Smith died, which should have made the change more urgent, but paradoxically made Heseltine, who had had a heart attack the previous year, the wrong choice, while Clarke refused to modify his pro-EU views.

As it was, Major saw off a challenge from John Redwood the following year — Redwood’s slogan was an accurate “no change, no chance”, but he was probably the wrong kind of change, amounting to “no difference” — and Major saw the parliament through to its bitter end.

Tony Blair was living on borrowed time after more than half of backbench Labour MPs voted against joining the Iraq war in 2003, and yet he survived for another four years as prime minister, including winning another election with a healthy majority.

Even Gordon Brown lasted longer than he might have done. He could have lasted only a few months if he had gone ahead with the election that never was — although I tend to agree with Ed Balls that he would have won it. He should have been brought down by David Miliband or Alan Johnson in 2009 or early 2010, but Brown, like Major, was allowed to serve out his time until the five-year limit on parliaments ran out.

When David Cameron formed his coalition government, the common assumption was that this rickety construction would soon collapse. When he made it to the end of a five-year term, the common assumption was that a small swing to Labour would make a Labour-led coalition possible.

There was a small swing from the Conservatives to Labour, but there was also a big collapse of the Lib Dem vote to the Conservatives, and Cameron carried on as prime minister until he came to a full stop a year later.

As for Theresa May, I was as convinced as anyone on the night in 2017 she lost the majority she inherited from Cameron that she would have to go the following day. Or the day after that, at the very latest. Two years later, she was still there.

Now we come to Boris Johnson. When he became prime minister, there were 100 days until the deadline he had set himself for getting Britain out of the EU.

I ran a sweepstake on how long he would last, and most entries assumed that he would fail and could therefore become the shortest-serving prime minister since George Canning, who died in office after 119 days.

As is so often the case, the popular wisdom was half right, but got the important part wrong. Johnson did fail to get Britain out of the EU by 31 October 2019, but that didn’t end his premiership.

Instead, he went on to win an election and get Brexit done in a way that suggested he might be prime minister for at least another decade. Indeed, it was only last September that The Times previewed the Conservative Party conference with the front-page headline: “Boris Johnson eyes another decade in power.”

That seems a while ago now, a hallucination induced by vaccine euphoria after the country had opened up and the economy seemed to be bouncing back so fast it was causing shortages.

This week the front pages were more like “the end is nigh”. On Tuesday, more of Johnson’s backbenchers voted against him than for him on the law to require proof of vaccination or a negative test to go to a nightclub.

Once you exclude the payroll vote — ministers and parliamentary private secretaries who are required to vote with the government or lose their jobs — and abstentions such as Theresa May, more Tory MPs voted against their government than for it.