I don't think it is 24 hours but once per day, I got my last coins yesterday at around 9PM, when I woke up this morning at 6AM I opened the app and got some defenders back with my 50 coins again.
But now the feeling is I won't do any gyms until this night because I can't get more coins and gyms are more tedious and cost more than before so why bother?
I don't like this system at all, gyms will become stagnant again very fast because there is no reward to take gyms down, even less than before...
