Various playthroughs by developers and QA estimate 15-20 hours, depending on what kind of person you are, eg. taking your time or rushing in really fast.
If you play on consoles you should be happy that it's one of the rare cases of games that play at 60 frames per second instead of 30, which means motion is much more fluid, and controls are very responsive.
Gameplay? See for yourself, http://www.youtube.com/bethesdasoftworks#g/c/81DA4873A6D0487A.