The Help states: "The I/O Wait Time alarm becomes active when the average I/O wait time for any database file exceeds a threshold. This value is taken over a specific number of background collections."
It appears to be a moving window alarm. The parameters of that window, like how big, how often it slides and how often and how many collections in that window.
This average is a calculation taken over 5 collections. While you are on the home page it will collect at 15 second intervals, or it is collected in the background at 1 minute intervals if you move off of that page.