From what I have read, the island events you are referring to come up on an average of 1 per 20 hours of playing time.
Of course it maybe more, maybe less but that has been the average stated.
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"Things to remember. . ."
"First, you forget names. Then, you forget faces.
Later, you forget to zip up your fly after exiting the men's room.
Finally, you forget to zip it down."