February 1. A few months ago I thought Donald Trump was a joke, and now I see him as an unstoppable juggernaut. Read this month old reddit comment about Trump's mastery of the media. I would go farther and say he has an intuitive understanding of mass psychology, and he's been laying the foundation for this run since the the 1990's. Because he has established a persona where people already expect him to say ridiculous things, he's gaffe-proof. Other candidates have to walk a tightrope between boring the voters and alienating them, while Trump is walking a highway where he can be popular and offensive at the same time. Somehow he can play the strong leader and play the clown. You can read more about Trump's powers in several smart posts on Scott Adams' blog.
Assuming it's Trump against Clinton in November, I see this as a repeat of 1996, where Trump is Bill Clinton, polarizing but charismatic, and Hillary is Bob Dole: unlikeable and unlucky. And Trump can easily rebrand himself as a moderate, because he has a long history of being a moderate before he talked like an extremist to win the primaries.
I don't think President Trump would ruin America, or save it. I would expect him to propose a bunch of simple-minded reforms, let congress rework them to fit the system, and where the reforms work he'll take credit, and where they fail he'll blame congress. The big doom scenario is if there's some disaster that shuts down congress, Trump takes temporary unchecked power, it goes to his head, and he doesn't give it up.