Lightning Returns: Final Fantasy XIII takes place over 13 days, according to a leaked interview from Famitsu (via Andriasang). Each day runs the equivalent of one or two real hours, though the scale can change. You can also increase and decrease the "life" of the world by helping people or clearing quests. The "world driven" game has players consider how their actions will affect this time-limited world, as opposed to the "story driven" FFXIII and the "player driven" FFXIII-2.
Because of the relatively short time limit in the game, it's meant to be replayed and cleared repeatedly. But that's not the only time-related weirdness: aging has stopped, so that nobody from FFXIII-2 has gotten older or died.
Mechanically, Lightning Returns now uses an Active Time-style battle system, with new elements like free movement and attacks based on abilities mapped to buttons.
Meanwhile, in official media, Square Enix has opened its American site for Lightning Returns, featuring multiple subtitled videos from last week's presentation. Above, design director Yuji Abe describes the time-based concepts.