A year at best.
People keep waiting for The One Thing(TM) that will let the bottom fall out and Trump suddenly has a 20% approval rating (which you ninnies will still wring your hands over because it's too high, which, yes, but that ignores the reality of the political factions in our country) and Paul Ryan starts drawing up the impeachment articles. It's not that easy.
The way to stop Trump is by romping the 2018 elections and getting a Democratic Congress. Then he's either impeached or we kick his ass in 2020 with a better candidate and a more unified left. Hope for anything else and you might as well pray that it starts raining bacon tomorrow.