Spoilers for episode 1:
By the end of episode 1 of Gakkou Gurashi, we know that some sort of zombie apocalypse has happened, and that the girls are trapped in a section of the school that they've managed to barricade against zombie attack.
How long prior to episode 1 did the zombies first start appearing? Given their rations situation in episode 2, anywhere from days to months seems plausible.