From my ETOPS manual:
"
PERIOD OF VALIDITY: At each alternate airport, the dispatch weather minima must be ensured during a certain time period. This period of validity starts
one hour before the earliest Estimated Time of Arrival (ETA) at this airport and finishes
one hour after the latest ETA..."
The earliest ETA at an alternate airport is defined by the departure time plus the normal flight time to reach the maximum diversion distance circle centered on this alternate airport plus the normal flying time to reach this alternate airport considering a diversion from the edge of the circle at the normal cruise speed and level"
The latest ETA at an alternate airport is the departure time plus the normal flying time to quite the maximum diversion distance circle centered on this alternate airport plus the diversion flying time at LRC speed and FL100 to reach this alternate airport considering a diversion initiated on the exit edge of the circle.
However, from a practical point of view and in conjunction with the Computerized Flight Plane (CFP),
the diversion point will be taken as the Equitime Point (ETP) between the relevant en-route alternate airports."
I know...it's not the simple rule you were looking for !
I also have a nice figure that explain very well the period of validity...but unfortunatly at this time I have no scan available!