With rules explantions, we took about 3hours for a first 4-player game.
I was never bored so I didn't mind at all.
Our first game took a while (and we didn't complete it).
Our second game (a three player game) took about an hour and a half - with bunch of the time at the beginning remembering how to play.
If the time limit is a major issue then reduce the number of rounds you play. You likely will not do many of the Black missions but that is okay.