Depends on if you are a completionist or not. I am, so its taking me about a day per level to do absolutely everything as far as challenges. Mind you, I am a very meticulous player and never rush through a game anyways.
If you run and gun, you could beat it in a few days. --- Retrouprising.com - Just about the best video game site ever made!
I split my time between this and AC3 and skyrim. Got this at Christmas so I think I beat it about 2 months ago, but I need to go back and get the rest of the playstyles/disguises and contract stuff. --- Gamertag: AccursedVenom