I cannot see how some reviewers have completed this game in 15 hours, and more importantly - why? There is so much detail to this game, and just because one has explored an area does not mean to say that it will not be slightly different a month or so later. This game is about having fun and discovering hidden missions to accompany the blatant ones. One could, for example, spend 15 hours just finding all the cameras and destroying them, one of the game's many parallel but non-essential objectives.
I would say that at least 24 hours has got me 39% complete. I have re-played some mini-games despite getting all three medals. If I wanted to complete this in a short space of time, I would have stuck to Sudoku's. I am having such fun with this game that for me the sub-missions (taxi driver, police, driver, fire driver, street racing league) are as much fun as working on the core objectives.
For the tenacious and patient players among you, this is great value for money and the game learning curve does not limit the continuation of play as I found with Zelda-where if you cannot figure out a certain sequence you cannot progress onwards.