Sand is impermanent. Over time it is swept by the wind, dug through by wild animals, and tidied up by groundskeepers.
Any community member who notices a sandbox post that's gone one week1 without progress2 should leave a reminder comment, also mentioning that it'll be deleted in a few more weeks, if no such comment already exists. Don't skip this step.
Any community member who notices a sandbox post that's gone 30 days without progress and had such a comment for at least one week should flag the post for moderator attention. If the previous step didn't happen, do it now and come back in a week instead of flagging.
Somebody updates the sandbox documentation so this doesn't surprise anybody.
1 I see no reason to tarry with this initial check-in; if it's been a week, time to remind the author that he's still got something waiting in the sandbox. But we give people a month before we actually delete things to allow for attacks of life, major reworkings prompted by comments, etc.3
2 By "gone without progress" I mean that the author hasn't edited the post and has not asked the question on main. (If the question has been asked on main, the sandbox post should be deleted for other reasons.) If the author is answering comments with comments instead of edits, ask him to edit instead. If other people have edited the post but the author hasn't been back, that doesn't count as activity. The purpose of the sandbox is to help authors prepare their questions, so if the author seems to have walked away, we don't care what other people have done.
3 Moderators and the community are not bound by this timing, and this is not a guarantee to post authors. If an obviously-problematic post is made, it can be deleted for cause same as with anything else on the site.