Can do, depends on your play style really.
AFAIK, took me 110 hours to complete base game to 100% completion. A quicker run through of main stuff I imagine could be done in much less time, half that.
The initial legacy of first blade adds about 15 hours or so and Atlantis DLC (only 2 of three parts are out yet) add new areas entirely so will take a bit longer.
Right now Uplay is showing 184 hours for me with remaining 2 parts of Atlantis DLC still to be played, so yeah for me at least 200 hours seems around the number I will hit, but that will mean having done everything, stopping and doing random things etc.