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EDIT 2: I was rewatching the zombies segment of COD Next and when talking about the Aether Storm they never actually said that's the limit, in fact they're pushing the whole "player options" thing, saying stuff like "Do we go into the storm to cleanse it, do we avoid it, do we get out of dodge" etc. From what I can gather they are definetely encouraging you to exfil within those hour long sessions to get better gear for more progress, however nowhere in that section did they ever say you are locked to that one hour, it's up to the player what they want to do, corroborating the Aether Storm info written below.
EDIT: TLDR; The timer does not indicate the end of the game but when a debuffing storm engulfs the map and makes zombies stronger, you start with a maximum of 6 players squad but you can assimilate every other squad with no limit.
Source: MWIII Vondy on the COD discord, who claims that CodeNamePizza divulged this info in his 4 hour and a half COD Next stream.
CodeNamePizza visited Treyarch shortly before COD Next and got some zombies info that NoahJ and others either didn't know or didn't play enough to find out.
This info is:
The 1 hour time limit IS NOT A TIME LIMIT FOR GAME OVER! It is the time before the Aether Storm engulfs the entire map, however, the storm does not kill you, it applies debuffs to you if you lack a gas mask but you won't take damage, and the zombies will be stronger inside of it.
We already know that the tiered zones are basically like rounds, so think of the Aether Storm engulfing everything as Round 50 or something.
He claims because of this, matches can at least (that means minimum) be stretched to 2 hours, but skilled players, players that find exploits or areas and have good weapons and most likely world record holders can probably stretch it to multiple hours.
CodeNamePizza has also said Treyarch devs insinuated to him that we would be getting round based in some form in MWZ to a limited extent from a small Treayrch team collaborating with an Infinity Ward branch in Austin.
He was also told that assimilation has no cap to it, you start with a 6 player squad maximum, however, you can assimilate other squads with no limit, theoretically, one could put together an entire 24 player squad.
The reasoning why it isn't a 24 player squad by default is because it'd make it harder to coordinate, revive, do quests etc.
If you're planning to take on huge bosses you can assimilate any squads you like and work together without ever leaving the initial lobby, gotta say this is cool!
I saw a lot of people complaining about the timer and wondering how the assimilation would work so I thought I'd share this info here too, mostly the timer is what people wanted to know as well as round based.
Hope this helps.