I experimented with this and confirmed: Mentors who get into the NN channel, stay in the channel no matter how long they are afk. (AFK'd overnight, zoned before going to work, and got in the channel and stayed there until I came back from work.)
By looking at the list of players, it's clear that a large chunk of the mentors on the NN are afk at any given time. That means that mentors who are logging on, and active, are blocked from joining the channel because the max of 100 has been met.
I think that a timeout of the channel at 1 hour of afk would not be unreasonable for the mentors. That way, players who have gone inactive for extended periods of time during the day can get booted in favor of players who are actively on. But a mentor who needed to take a dinner break would also not be booted as long as they came back within an hour.