My prediction.
Muller drops the bomb immediately after the Mid Voting polls close and before the results are in.
it is pretty clear that after the Mid Terms (win or lose) Trump will fire a whole bunch of people around the investigation and hope his new AG will 'Temporarily Pause' the Muller investigation while they appoint new investigators to look into whether the investigation is being done fairly or not. That is the tact the Repub's are setting up so they can say 'the investigation is not canceled. We have just paused it while we investigate the investigators', which would never end and would just see the whole thing die.
So Muller, knowing this will file all his charges for the Public to see so regardless of what happens to him, no one, including a new AG will be able to avoid dealing with it. As ballsy as the Repub's are in defending 'The Don' from everything and ignoring all his wrongs, it should be near impossible to 'Temporarily Pause' an investigation if charges are filed accusing the President of direct crimes. Those charges will have to be dealt with first and foremost.