Generally, if you're within 10 - 15 minutes either way (early or late), the CM will just put it into the computer as though you were there right on time, and you'll be in the queue to get the next available table. We've never had trouble within 15 minutes... they understand you may have waited a little extra longer than you thought in a line, or your child may have had to make the 300th potty stop of the day.
However, much earlier than that, and there is a reasonable chance they'll just treat you like a walk up. If there is room, they'll seat you, but anyone who shows up with a ADR earlier than yours is likely to be seated before you. And yes, much later than that, and you're likely to lose your ADR. It totally depends on the CM, of course. Some will try to work with you, some won't. I don't think there is an official window, but I don't know for sure.