I'd put it between 6 months and the mid-term elections. He'll be gone well before 2024. It wouldn't be surprising if Harris did not contend 2024, depending on how well she's polling before the next election. I think she was the preferred primary candidate of... 5%... of Democratic voters. We could be looking at two of the least popular Presidents in American history.

She has one obvious advantage over Biden in that the various facets of her identity are associated with powers/privileges that Biden simply doesn't have. That is, that Democrats will be forced to be hush about Harris' failings simply because any criticism of her whatsoever is a combination of 'racist' and 'sexist'. On the other hand, those like Biden occupy the lowest rungs of the American power structure. Nobody cares about 'ageism' and that's the only -ism he could realistically accuse his opponents of being. I don't see what he can really hide behind when the criticism (of which there will be a deluge) really begins hitting him.

I don't even think he'll resign willingly. I think he'll be sidelined out of power, with Kamala rapidly becoming the de facto President (e.g. the one that the media actually want to talk to and whose face is shown on television) and de jure before 2024. Eventually he'll simply be told that his past 'racist' comments and his cognitive decline make him unfit for office and that he has effectively been ousted by the progressive faction. Like an informal no confidence vote. People will just stop listening to him. And that process will be accelerated by the fact that his popularity will be far lower than that of Trump. Unlike Trump, his own 'side' doesn't even like him. At least Trump could rely on the Republican base. The Democrats know that this guy has become a serious liability and that they need to be rid of him fast. He practically embodies everything that Democrats also hate about Trump and he's suddenly developed extra weaknesses in the form of his idiot son and his mental state. They accepted him because he presented himself as a veteran, 'Old Guard' candidate who could dupe enough Republicans into flipping sides at a time where they're about to deliver the coup de grace and desperately need corporate and White voters (i.e. those most likely to oppose them) to remain passive a little longer. You don't want your targets squirming around, but to meekly accept what you're going to do to them.

Where I might disagree with your comments elsewhere is on the matter of the Democrats developing a supermajority. I think they know that they can defeat the Republicans permanently with a combination of if they, firstly, allow Puerto Rico to vote in the Presidential election, secondly, allow illegals to vote (by changing the law or by naturalizing large numbers of them) or, to a lesser extent, thirdly, have the election decided by popular vote.

The first option seems a dead-end as it harkens back to the whole colonialism/imperialism/racism thing. The third wouldn't guarantee victories if the Democrats performed so poorly that millions flip sides. The second seems like the obvious way forward and that's why we're already hearing that they intend to do this. Unless the Republicans find a way to prevent this, they'll easily have 2024 and beyond simply by convincing naturalized illegals that another Republican Presidency would reverse it. Desperate to remain in America, they'll be a practically 100% loyal voting bloc.