There are no statutory deadlines. A candidate, president elect, and president can resign any time they please. Not joking, that is the way it is, especially for a presidential election. When you pull the lever, you are voting for a party. The party chooses electors, and the electors vote for president, in December.
There are ballot access deadlines and they will all be past soon. In Illinois the state law was just changed so that the President would be on the ballot as the nomination was going to take place after the original deadline had passed.