Yeah, most gyms do turn over fast. This one actually had turned over from Instinct to Valor to Instinct in the morning and I pretty much was like "I am taking this one down" at lunch and by the time I left work, two other Team Mystic pokemon had been dropped off.

I really wish there was a way to check Pokemon Gyms long distance and see what's going on with them because I'm terribly curious about what's going on there.