I don't see the republican majority acting on this even if Mueller comes back with a long and very strongly supported list of illegal activities by Trump, so the "when" part is open to discussion. My best bet is that the fall election will be over this very issue: Impeach or not.
As to the what, I am not doing the investigation but at the very least, Trump has openly admitted to Obstruction of Justice when firing Comey, so at minimum, that will be one charge. There are many other possible charges that require little imagination to expect, including violation of the emoluments clause and abuse of power (the misdemeanor part of "high crimes and misdemeanors", which means "malfeasance" in today's language.
Collusion might, or might not be in the mix. There will almost certainly be evidence for money laundering, but those acts predate occupancy of the White House by Trump and thus are not, strictly speaking, cause for impeachment. They will be cause for arrest and trial once out of the White House though. I believe that this is what has always frightened him about the investigation and is the most probable explanation for never releasing his tax forms.