Your replayability is determined by how much you like the game. It's not like there's much left for me to do in FFVII or X, but I can play them over and over and never really get sick of them. Compared to XII, which is huge and has ridiculous amounts of content but I've not even finished because I just don't like it. XIII isn't really much different in that regard, except that I could see the first half of the game being reaaaalllyyyy tedious to play through more than very occasionally.