Aggron is still in there
Answers
Dropped a Nidoqueen into an out of the way church. Was there 109 days 20 hours, from fall 2017. Never put another mon into it, easily gold, still my #4 gym after almost a year.
Put a Xatu in a gym in Tbilisi, Georgia (in Asia, not southern US state), berried it a few times, lasted 40d 7h 5m.
Those are my longest.
(Wouldn't be surprised if a spoofer found this post and knocked it out, though.)
Good luck.
Under the new gym system it is 32 days at a national park. The problem is lack of cell service, I've gone there several times and you really have to get lucky to get a signal by the gym.
Under the old gym system I had gyms lasting for months as the wife and I would go out of our way when we traveled to find level 7 or 8 gyms of our team in remote areas and prestige them up so we could get our 100 coins a day.