They last for as long as HCS puts them up. The last 2 lasted for 24 hours but I believe there have been Legendary Events which have lasted longer (before my time of joining). The time they last is included in the announcement HCS make on the home page.
They are not pre-announced so you will only know about it when it begins and HCS post a notice when it has begun.
Yes, if you are in the World area you will be notified as the message will show as an update on the home page
