Didn't they say that you could gain more time by doing various quests, or was that just speculation? Since the majority of this board are self-proclaimed ADHD kids with a touch of OCD, they'll probably do every single quest that pops up and gain more and more time. So don't worry, the game will probably be long enough.
The time gets set back by defeating monsters and completing quest objectives. Though moved forward by causing "miracles" according to Yuji Abe's presentation.
It might be longer or maybe even shorter than you'd expect depending on how much each action effects the movement of the doomsday time and whether like Majora's Mask you can reset it or it's automatically reset if you aren't ready for the end.
I wonder what the Miracles are supposed to be though? Limit breaks at the cost of time?