From the Commission on Presidential Debates:
All debates will be moderated by a single individual and will run from 9:00-10:30 p.m. Eastern Time without commercial breaks.
When will they be sitting down?
The key part of that statement is: "without commercial breaks." That only means no commercials. It doesn't mean they won't take a couple of breaks to allow the candidates (especially Hillary) to regroup. The moderator will then send it back to "our panel in the studio for some analysis of what we've seen so far" while the break is in progress.