Niall Ferguson writes:

I wish Andy Burnham the best of luck as he takes his turn atop the greasy pole. If he lasts two years, then dinner—at Peterhouse high table—is on me.

Why Britons Really Regret Brexit

I interpret this that he doesn't think there's a big chance Andy Burnham will last that long.

This market will start it's timer when Andy Burnham becomes PM. This is the time estimate for that (Copilot):

Andy Burnham’s route to Downing Street depends on winning the Makerfield by-election, which he has already secured, allowing him to return to Parliament

Once sworn in, he could trigger a Labour leadership contest by securing the support of at least 81 Labour MPs (20% of the parliamentary party) and backing from constituency parties or affiliates

If no other candidate challenges him, Burnham could be elected unopposed and installed as Prime Minister as early as mid-July, with a formal transition by September

If a contest occurs, it could take several weeks to months, depending on nominations and party voting procedures.