How long will he last? Photo: Victoria Jones/PA Wire/PA Images

When you’ve waited for a job your whole life, you’d hope to have the time to give it a proper go. But today, as the boy who grew up wanting to be “world king” stands on the threshold of Downing Street, his many detractors are chattering about whether Boris Johnson could just prove to be one of shortest-serving prime ministers in history.

So what are the benchmarks, and is there really reason to think that the blond bombshell could blow itself up in record time?

Overall, British prime ministers have not fared too badly in job security terms—there have been 54 of them since the inception of the office 298 years ago, which makes the average tenure about five and a half years. But for several of them this was split up with gaps across different administrations. And so the average term is shorter, coming in at a shade under four years.

With three years to go until the next scheduled general election, which is nowadays supposed to be fixed by law, for the summer of 2022, and now with a two-to-one mandate under his belt, Johnson could, in ordinary times, hope to get pretty close to that benchmark without doing anything special at all.

The times are, however, the opposite of ordinary, and not only because the referendum upended over 40 years of trade and foreign policy. Now, it could just be that Johnson, a chaotic character, will thrive in chaotic times. But the old parties are splintering, both in the Commons (where there are currently 16 independents on top of the five deserters in the endlessly-shape shifting Independent Group for Change) and in the country, where the polls reveal the steady two-party politics at the start of the year has suddenly and dramatically morphed since the spring into an even four-way tussle. Rather than looking only to British history for guidance on how long a prime minister could last, should we also look to other parts of the world in eras of tumult? Like, for example, the French Fourth Republic, which in just under 12 years got through 21 premierships, some of them for tenures that were counted in days.

Today—as in post-war France—the very ground rules of politics are in contention in a way we’ve not seen in modern Britain.…

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