Like corpses. And corpses don’t tend to be particularly strong or fast once they start rotting. Unless there’s some sort of magic involved to stop the decomposition process, they’ll likely all die out as they rot. The fresher zombies will take a few weeks to become inert, but that’s a blip in the radar in the grand scheme of things.

But the bigger issue then becomes what caused it. If it was something like a prion disease, (like Mad Cow Disease) then you’re likely fucked even after the zombies all rot; Prions don’t decompose, and (if the zombie outbreak was big enough) you’re almost guaranteed to catch it via dirty soil exposure (either directly, or via tainted produce that was grown where a zombie died) eventually.

My head cannon for “realistic” zombies. is Mel Brooks’, the writer of 'zombie survival guide and ‘world war z’ (no relation to the movie or the game) esque zombies.