I'd put money on May staying for ever. There are perhaps 50-60 hard Brexit headbanger Tory MPs. That's enough to trigger a vote of confidence, not enough to win one, but probably enough to ensure that there's at least one hard Brexiteer in the final two candidates offered to the membership. The assumption is that the membership will vote for a headbanger. So the majority of the party have nothing to gain - and everything to lose - from ousting May, and so May would likely win a parliamentary Conservative party vote of confidence.
Whatever outcome we get - chaos, EEA+, people's vote 2nd referendum or a cake deal - it will be voted for by Parliament. Yes, the government will lose some crucial votes. It may well have a solution imposed on it by MPs. But that doesn't trigger another election unless there's a vote of no confidence in the Govt, and it's very unlikely that the ERG mob would ally with other parties to bring down the government, risking PM Corbyn - and deselection.
So the most likely scenario is that May goes on her own terms in 2021, by which time it's likely to be more obvious what's going on with Europe, and the membership could be hoped to have electoral success on their minds, rather than Brexit.
Anyone want to bet against this?