According to research provided by some place called Argonne National Laboratory (never heard of ‘em), it would take zombies just 60 days to completely decimate Chicago. The Windy City can hold its figurative head high, in that it managed to last longer than New York did in THE STRAIN television series. (Granted, the vampires used a nuke, so that might disqualify their efforts from consideration.) Presuming the zombie apocalypse to be spread by a virus, and that anyone bitten by a zombie and surviving would subsequently die and return to un-life as a zombie, the computer program utilized by Argonne estimated that the city could hold out for a couple’a months, give or take, before it became the exclusive domain of the walking dead.
Argonne studies viral outbreaks from diseases such as Ebola, so the thing with zombies was just them having a bit of fun. They even took into account that humans could be trained to dispatch zombies, which would, if enough humans became skilled zombie slayers, prolong the existence of the city. (I’m not sure why anybody would want to, if it was full of zombies. Seems like moving would be a more attractive option.) The study did fail to take into account, however, the scenario of zombies getting hold of a nuke. Then again, it’s doubtful, if zombies did have a bomb, they’d know how to detonate it.