I think it's actually more likely that he will resign because being president is no fun and a lot of work health reasons and again Republicans will praise him for being so brave to step down. And we get president Mike Pence.
Assuming that there is no rhyme or reason to the dates that Congresses choose to impeach on (e.g. lack of things like "never impeach on a Friday"), he has a P(A*1/2,923) chance of winning and P(1-A) of losing, with A being the probability that Trump will be impeached, whatever that is estimated to be (for example, PredictIt has a market for the next POTUS being impeached in the first 100 days of office, currently sitting at 5%, which seems pretty overpriced).
I want to say Dec 31st, 2019. By the Republicans so that they can simultaneously say that they allowed Trump his full term and that they removed him from office. Pandering to both sides of the issue at the same time.
But, my actual guess will be: April 20th, 2018. Coincidentally Hitler's birthday, but done so Republicans can run midterm election campaigns on "We Stopped Trump."