What will happen if the time runs out?

[Johnny]

I mean... you have to do everything under that time? Or when the time is over its end game whether or not you completed the game?

[DrNewcenstein]

You have to do as much as you can within the 72 hour overall limit. For missions, you have to complete them before their timer bar expires.

If you do not do anything in the 72 hours, but just lounge around the Security Room or the helipad, you get back on the chopper and leave.
If you fail to complete any given mission, you simply fail that mission. If it's part of the main story, you get "The Truth Has Vanished" and can continue to play until the end of the 72 hours. If you do nothing else, or otherwise fail to return to the helipad on Day 3, you "die" and can either load your last save or start over.

If you do not complete the story, but rescue survivors and defeat psychos, and return to the helipad on Day 3, you get a certain Ending rating and the game ends.

The goal is to complete all Story missions, regardless of how many survivors you save or pictures you take or psychos you defeat, and unlock Overtime Mode, where you get the True Ending and the full story.

You do not have to (and cannot) do "everything" in one playthrough - save all survivors, defeat all psychos, take all PP sticker pictures, complete the story, get OT mode, unlock all unlockables/achievements, max level, zombie genocider, etc.
It was meant to be played repeatedly, especially once you reach max level. Take one playthrough and focus on something besides the story. Rescue everyone, take all the PP sticker photos, get whatever achievements and unlockables remain.

[Johnny]

thx for the infos!

[thattroygriffin]

Old post but you CAN do all scoops/survivors/psychos in one run, and even get max level if you use a world news magazine and kill the special forces in the end game, but it's hard. And as for PP stickers you have a lot of time during certain parts of the game. Two biggest ones are waiting to meet isabela in North Plaza (not that much though) and right before the game ends when the military first arrive until they clear out which you need to wait to happen in the hideout with Isabela (but you can kill time by doing stuff in the meantime). You can't do all the achievements during this time but you can grind out some and get most if not all the PP stickers (they're also easy to get as you play the game too)