Several subs need new moderators

I have found some subcommies that have no active moderators. They will have one or two moderators appointed, but they are either inactive or sometimes have deleted their accounts. Communism101 is one example (only mod has deleted account). Other cases, I have found the moderator(s) without posts or comments for several months. If something should be done about moderation of communities with inactive mods, at what point can we agree a mod is inactive?