I think he means it
He says this over and over again. At some point I think people have to consider that he's actually thinking about it.
I think people believe he's trolling and of course that's part of it. But he is a salesman who believes that loud and annoying repetition is the best way to advertise. (Think used car ads on late-night TV.) So I don't think it's smart to dismiss this out of hand. If the election is even slightly close anywhere I have no doubt they will contest the results. And if it turns into a political shitshow as it was in 2000, I could easily see him refusing to leave and we simply have no real mechanism for removing him without his cooperation.
This could happen in 2020. But if he legitimately* gets four more years there is no telling what they will have done to make this "legal" by 2024.