if a member goes AWOL how far back can they delete content?

Deletion is a permanent ability.  

This is a permission for each membergroup that you can define per content item. 

One thing you can do is, instead of allowing delete, to allow your members to be able to edit a content item up to a certain time.  That way they can remove or empty an post, but only up to X days ago.  

Go to ACP > Members > Groups > edit the group > Content tab