The Mueller investigation is progressing, it's slow going perhaps, but that's the nature of these things. In 6-9 months they might have enough to make impeachment the only real option. If his approval ratings among republicans drops by a chunk the political motivation to impeach will increase significant, even barring clear findings from the investigation.
The political swing is also looking significant. If the Democrats manage to take control of the Senate it would be a game changer for Trump and massively increase the chance if impeachment. A solid democratic candidates in 2020 looks a very solid favourite right now if up against Trump.
Not a betting man on such issues, but I think the chances of him being gone in a year are higher than him being there for another six.
I hope Trump stays the full-term and runs again, against Sanders who would smash him. Pence would be a much harder opponent for Bernie Sanders. Unless there are any shenanigans, or Sanders chooses not to run, he will almost certainly win the Democratic nomination. He's far and away the most popular politician in the Democratic Party (the last polling on it that I saw, Sanders is the most popular politician in America). By 2020, it's not unrealistic to think that there could be President Sanders, with a Dem controlled Congress.