I imagine that the scenarios are of varying difficulty, and thus, length.
I also imagine that to some extent, the number of players varies the play time as well.
Any estimates from those that have been able to play on how those two interact?
My experience so far is that in general, more players makes the game take longer. In every turn, 4 more command cards are drafted, so the team's "power" increases at a similar rate regardless of number of players, but in a 4 player game, twice as many players take their turn compared to a 2 player game.
That said, I think different scenarios are easier / harder for different numbers of players, and that makes a difference too.
Difficulty can shorten games as well as lengthen them... if the mission is very difficult, losing quickly can occur, which results in a short game.