A lot can happen between now and 2020, but assuming the country is still functioning, I expect Trump to get bored and decide not to run for reelection. I think what he wanted was to become president. I doubt he will enjoy the actual work of the presidency.

The question is, will the Republicans pressure him to run again? An incumbent president is a powerful asset, regardless of who it is. They'll have to weigh their personal dislike of Trump against the political liability of starting from scratch with a new nominee.

That is, of course, assuming his administration isn't a total train wreck. Then all bet's are off.