According to the new rules for leadership elections, Corbyn stays until he either resigns or the members/affiliates kick him out. Though he might receive no confidence motions, threats and sharp critique throughout his tenure. Indeed, this in part is what's pushing reasonably capable MPs like Sadiq Khan and Burham to move for mayoral positions. It's a good way to wait out the current soul searching era within Labour. The longer Corbyn vacillates in his bubble, the more of them depart the shadow cabinet and the backbench, going forward, imho.
Doesn't mean it's a good thing.
Comrade Corbyn has several problems:
1) A London socialist, from an era before this meant Blairite, but one nonetheless; and that will struggle to win lost voters in the North.
2) Hates the media, at times justly, at times to the point of tinfoil paranoia; the media hates him back.
3) Divides the party and their strategy.
4) Warm, approachable and kind, but has so few policies to set him apart from Ed max rhetoric, it's not funny. Some stuff like the electoral reform can get off the ground. More local power and spending is always popular (but not enough in itself). His 'new economics' are in bad need of fleshing out.
5) A total turnoff for wavering Tory voters Blair could win, who will be becoming more and more important as we further diverge from Scotland or separate from them.
6) His idea of campaigning is to talk inwards, even though he could probably do what Sadiq did in London more often and with good enough results, pending some practical improvement to the other points above.
7) Not a great opposition leader or Commons performer. His new politics is only workable if either the Speaker or the other team play along.
8) Shutdowns under hostile questioning and conditions, looking always for external factors to explain the hostility.
I can go on, and the fact that my mental list is far longer than the equivalent would be for Blair, Brown, Balls, and the like, is telling and I'm not even a close follower of Labour politics. What a supporter outside Momentum must be feeling like? I don't know, but they can't be all happy bunnies.
So, candidates aren't obvious: Burham, Khan, Blue Labour Woman, Benn and a crop of people that backed away from running last time, like Chuka. Will be easier to tell after the next general election.