There's been a zombie outbreak. Only you and the people inside the same building remain. How long will you survive in the building?
It's 11:30 AM and I'm at work. There's about 700 of us in the building...
Assuming the only issue is zombies on the streets, we're probably good for a day or two.
The building itself is relatively secure - just a couple entry points that can easily be closed/locked/barricaded. There's a wide-open roof a couple stories up where we could wave for help or pelt the zombie hordes with office products.
Assuming no breakdowns in infrastructure, we'll have plenty of water for the foreseeable future. Plenty of heat. And even if the electricity cuts out - we've got a full-building UPS and a generator, so we can stay up and running for a couple days without even trying.
The issue is going to be food. We've got a cafe, so we're good for a couple days... There's a rooftop garden, but that's not so helpful when it's 3°F outside... And 700 people are going to eat a lot.
I figure we're going to be out of food by the end of the week. And I mean out of food... We'll have resorted to drinking ketchup and mustard for sustenance before then.
The question then becomes: Just how dangerous is it to go outside?
I figure those same cold temperatures that render our rooftop garden useless are going to give us an edge over the zombies. A couple sub-zero evenings like we've got in the forecast this week and they'll just be meatsicles.
There's several restaurants within about a block of the building. A couple convenience stores a little bit beyond that. And there's a couple grocery stores less than a mile away. Lots of food/supplies within walking distance... If you don't get eaten on the way.
Assuming we're able to get out for food... Medical supplies are probably also readily-available as there's several pharmacies and even a hospital within walking distance.
Then the big problem becomes organizational/societal. You've got 700 people trapped in a fairly small area for an extended period of time. It won't be long before the day-to-day routine falls apart and people start getting short with each-other.
It'd probably actually be a good thing if the population decreased dramatically... Either because people are going home to loved-ones, or because they're getting eaten on their way to the grocery store... 350 people would be a hell of a lot easier to manage/maintain than 700.
If we're talking about a genuine zombie apocalypse, however... We're ultimately doomed. There's only so much you can do when the world is over-run with zombies. Society is going to collapse. Organized agriculture is off the table. We're going to see roaming bands of survivors scavenging what they can, when they can.