@[email protected] went on a campaign to find moderators and lock unmoderated communities a while back.
As far as I know, moderator seniority isn't such a big deal on Lemmy like it is on reddit. Mainly because the admins can easily overrule the moderators anyway. I don't think there is really any harm to leaving inactive moderator accounts on the team. All reports are sent to all moderators as well as admins, so it's not like inactive moderators are preventing admins or other moderators from doing what they need to.
However, if you would still like to remove them as moderators, I think a reasonable standard would be 30 days without any activity or response to PM. Just PM me whichever accounts you are concerned about and I can demod them if they don't respond in the allotted time.