Pop Up in Game
That is the standard notice given each time they release a game update if you are online during the update.
The actual amount of time you get seems to vary:
If you are NOT in an instance, the timer is 3 minutes.
If you ARE in an instance, you get 1 hour.
In any case, you don’t want to wait for the time to expire because then you are forced to try to download the update at the same time as thirty billion other people. Best to logout and login again as soon as conveniently possible.