How long can a pokemon stay on a gym?

Forever

Until someone knocks it out.

No limit as far as I know. People have posted screenshots of pokemons staying in gyms for months.

I've seen a gym with defending times about 25 days, one week later all times turned to negative

I have a Ledian that's been in a gym for over a month, and the clock turned negative a while ago. I'm curious to see what happens once it reaches zero in about 13 more days.

My other two defenders have been in their gyms for 11 and 3 days as well, haven't gotten any coins since Thursday.

someone in my chat posted a picture 100d.

A week is only long for a big town/city. When I visited my parents (2000 people village) it stayed a week too in summer and the gym is placed pretty much in the center right at the main road.

I think if you find gyms away from a city and the road than you make it gold.

I placed a Donphan in a rural location on October 5 and its still there >40 days.

My personal record currently. Somewhere in the forest where no street leads to.