We’re now in the pre-March 15 cycle. If has to happen sufficiently before 3/15 for voters to consider who is left. Dropping out typically occurs immediately after the primaries before.
All the candidates repeated yesterday they’re staying in for the 3/15 primaries. For it to work only one non-Trump can be in 3/15. It isn’t going to happen. Trump will be the nominee. And Clinton will be the President.