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Ranger anon here. Yesterday I had taken 14gyms and 13 of them were still mine when I went to bed. 9 hours later I woke up to see I still had 5 of them in and pokemon were giving me coins of values around 14 each.
If I can go out there and retake then this gym system will be pretty easy. Weird thing is the 5 gyms I still have are easily accessible in the park. I can assume a kid hasn't walked up to it playing the game yet, even if it is Friday.