Most pokemon games took me around 30 hours to beat. This is mostly because of grinding I would do between gyms and such. If you only fought trainers, only caught/trained 6 pokemon, never backtracked to reach previously unreachable items/areas, indulged in any side activities until post game, the story would probably be around 6-10 hours.
After that though is catching and raising lots of pokemon, breeding pokemon, and playing the battle tower variant this game has. That easily adds hundreds of hours. Then you have online battling, which gives the game infinite life, at least until a new generation comes out.