Honestly? I would imagine the main sequence to be about the same length as other Pokémon games. I have however noticed that the games in the main series have been growing longer, one of the most notable jumps being between GenIII and GenIV, which was a change in console much like the jump between GenV and GenVI. I would imagine X and Y will be longer than Black and White 2, most definitely.
The main increase I am expecting will be in the post-main story sections. Of course the main storyline will be longer, at least I think it will be, but I anticipate the parts which happen after the defeat of the Elite Four/the criminal gang/your rival being significantly longer and more extensive; it will be more fleshed out.
That is something I would definitely look forward to.