The casino opens every 4 hours from its previous opening. This opens the casino for 1 hour before closing for another 4 hours.
If you get online and want to know when the last casino event happened or when the next one might be, it is highly recommended to ask other players on the GTM you are playing on at that time, since most of the time there will be a few players who can most certainly tell you about the schedule and when it will next open on that specific GTM.