For the forum part of site in general (excluding web finds) my feeling is nothing should ever be deleted if someone "deletes" a post or thread, it should simply be moved to a hidden/admin only board. (That said, on the gallery side if someone deletes their own photos, then they should truly be deleted)

Say the web finds section had a 90-day expire time...
Could it be set to email the original poster of a thread with a 'warning message': something to the effect of "a thread you started will be deleted in XX-days" then they could move it a 'web finds-archives' section if they wanted it saved. Or something like that.

If we ever do delete anything, as far as the forum is concerned, it's usually moved to a 'deleted items' folder so that it can be restored if necessary. How long it stays there depends on several things; the content, reason for deletion, etc. But it's not often things are completely deleted, except for things such as obviously expired links that are never going to work again.

The gallery is a different matter as Coppermine doesn't provide an archive facility as far as I'm aware. But Patrick can usually recover deleted gallery posts from backups if necessary.