How often does a Transfer Market change its players?

By saying that, I'm asking if anyone knows exactly how much time does the TM need to add new players since your last visit? I'm desperately looking for new players in my defense because I only have 3 left. I'm considering using a timer to check every time the TM changes.

When looking for players, I generally refresh every 20 minutes. The transfer list doesn't show all players so it sometimes shows hidden players after refresh.