Corpses left exposed in a typical temperate climate will be reduced to skeletons after a year. The skeleton can take another 40-50 years to be reduced to dust. This all assumes insect and microbe life hasn't been removed by the apocalypse. Burial practices, fossilisation processes, and environment can extend these almost indefinitely.
https://www.aftermath.com/content/human-decomposition/
https://www.quora.com/How-long-do-bo...hey-are-buried