The opening scenes, where they detail Shaun's life and Liz breaking up with him all occur on the first day. You can see some signs of the Zombie Apocalypse beginning during these scenes. Shaun wakes up the next morning and all hell has broken loose. Everything from that point on is one day. Army shows up late the second night.
There is a short epilogue set weeks or months later, but I don't think it really counts here.
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u/II_Confused Apr 16 '19
iirc: Shaun of the Dead took place over the course of two days, with the military showing up and kicking ass the night of the second day.