If they're keeping in touch (excluding events where you know contact will be difficult from them) then I don't think I'd ever fully demote a staff member. I would probably just remove them temporarily into an LOA group during their absence just to make it clear to anyone who might not know that person is LOA that they are, rather than having anyone assume that staff member is possibly available to ask something or thinking the staff team just has an inactive person on it without good reason.
All depends on the situation for me. At most no longer than a month if I’m in need of active staff members. If so, I’ll either find a replacement or make sure to let them know to keep me up-to date on what’s happening with their situation. I’ve had instances where staff members would go on LOA and never come back.
Without word? Probably a month. If they keep in touch and let me know their situation ahead of time then I don't think I'd demote them unless they were gonna be gone for 6+ months or so.
It really depends on the situation at the time. If they needed LOA for a few months and they informed me, I would be happy with allowing that, if they kept me updated throughout I would be willing to keep them as staff until they could be active again like they were.
If they do not let me know, then I would say a month and I would demote.
For me, since a forum isn't really a form of employment (unless I'm paying someone), I'd ask how long they are going to be away, and if they exceed that by about a month or so, I'll just demote them from staff until they return and see if they want to be active again.
I feel like that is fair to all parties.
An inactive staff account could pose a security risk, though. So, I wouldn't recommend allowing it to stay inactive and open for too long.