There are no set times of the day that the events go live. All Assault Events across every realm, function on a 19 hour cycle of 7 + 12 hours. The Zone Assault Quest (ZAQ) and the Assault World Quests (AWQs) are live for 7 hours, at which point they expire.
How Often Do the Events Start Up Again?
12 hours after the previous event has expired, a new event will start up. This time it will be on the other factions continent, as each faction takes turn in defending their territory.
How Many Zones Can The Event Be Found In?
Each zone, aside from the faction capitals, can stage the event. Invasions will only occur in one zone at a time. They are chosen at random, but as the incursions switches between the continents with start of every new event. There will never be a zone that hosts the Assault two times in a row.
How Many Assault World Quests are There?
Each zone has multiple AWQs that could be triggered when it is invaded by the opposing faction. There will never be more then 5 AWQs to choose from during each incursion. They are randomly selected from the list of quests available to that zone. This means that the ZAQ for that zone, may have to be completed multiple times in order to do every AWQ for that zone.