By once a day, I mean that a game that is played in a "typical fashion" will see an Island Event about every 24 Earth-hours, on average. If you leave the game paused a lot, that will affect the frequency of events. In addition, the total hours played that is shown on the statistics screen includes all of the time that the game is paused, and is somewhat misleading, since you can't determine how much of that time the game was actually unpaused.
