For me it takes about 45 to an hour to complete one mission. I guess that I am a little slow at them or I just suck at them but I don't think I am very fast at completing them. How about you? How long does it take to complete missions for you?
If it's storyline mission, you're on target.
The storyline missions were originally designed to last as long as watching an episode of Star Trek.
If it's your first run through them, sit back, relax, and enjoy the ride.