I think the average is 5 minutes, though I never measured myself playing. Of course, I go at maximum speed since I know everything (I even made the missions myself). I guess in the beginning will take more since you need to learn what each card does, the basic mechanics, etc.
Differently from other multiplayer CCG games, the computer moves instantly. So you never wait long, and that's something to consider (for example in Hearthstone there are some SLOOW players and out of a 10 minute match, you can easily spend half of it waiting for opponent's moves).
I honestly believe so far the length of matches is perfect, not too short, but not too long, so you can keep reading the story. Like Loren, a player more interested in the story can set difficulty to Easy and play the base 30 story missions very fast and read the dialogues, while those who like the gameplay part can set difficulty to Hard and they'll NEED to play many side missions to "grind" to continue the campaign
