Depends on whether you do all the personal missions for your crew, but even rushing through I'd estimate 30 hours, more like 50 if you want to complete it all. Are you planning on importing your character into ME3, because that may influence whether you want to take your time with it?
Took me 27 hours first time round thats with all the Major side missions (Loyalty quests) there is another good 10 hours worth so of side quests tough. Can get through in 15ish if your really pushing, but will likely regret it as it effects the ending you get.
"Gentlemen, you can't fight in here this is the war room!"
Do not scan all the planets!!! It's a huge time investment for very little payoff. It's nice to have enough resources to upgrade things, but it's definitely not worth the hours and sore thumbs scanning everything.
I did it thinking I would get an achievement - nope.
The resources you have left over after scanning the planets get carried over to ME3 and go into your war assets, for what small amount they contribute. I liked planet scanning and shooting probes, because I'm sad like that, so I had loads I hadn't spent.