I don't think there's a road map for this game and the achievement guide isn't all that great either. The solo campaign is pretty short so you can just play through once on elevated and then clean up the rest of the secondary objectives on casual or whatever it's called.
The multiplayer takes a bit longer; you have to get up to 500 kills, win 30 team, 30 co-op, and 10 solo matches, lead your team in kills 10 times, etc...and bunch of the multiplayer achievements need to be done with 5 unique gamertags in the room.
Anyhow, I'm thinking 40 hours or so. I definitely spent way more time than that but I didn't boost any of the achievements until I was essentially required to (Solo veteran & demo expert). If you found a bunch of people to boost and were only going for achievements you may be able to do it in less time but i'm not sure.