How To AFK Prayer in OSRS

Using an inventory of bones at a Gilded Altar lets you AFK for 49 seconds at a time.

Gilded Alter

And if you were to do this at the chaos altar in the wilderness, its about a minute and a half of AFK time.

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Ensouled Heads

Reanimating ensouled heads can also be decently AFK with auto retaliate on, but less than 30 seconds of AFK time.