They're never gonna impeach Trump.
They might, if he's causing the party to tank enough to cost them the House and the Senate in 2018--especially the latter in a year when 10 Democratic incumbents are in states Trump carried, so they're looking to gain in the Senate.
And even if he doesn't quite do that, they may well ditch him before 2020, because there they could lose the Senate (having 22 incumbents to the Democrats' 11) and if things go really badly, they lose state legislatures just in time for redistricting. (Granted the Dems might not be able to do things quite as atrocious as the GOP did after 2010 if
Gill v. Whitford goes the way any sane democracy would have it go, but...)
But even if they just lose Congress for 2020, that still loses them control over federal redistricting, if the Dems are willing to go that far. (I don't think Congress' residual power over their own electoral maps has ever been exercised, but it's there, per Scalia's plurality opinion in
Vieth v. Jubelirer and not contested in Kennedy's concurrence or any of the dissents.)
Plus he keeps pissing off everybody in Congress.
I highly doubt Trump will finish his term. He's going to resign, be removed per the 25th Amendment, or be impeached.
EDIT: