2 hours is the estimated downtime, so if you're on one of the affected realms, I'd start checking back around 2:45-3:00 pm EST. We'll keep you up to date if maintenance is extended. Read on for the list:
UPDATE 2:03 pm EST: Anub'arak is now listed as the only realm affected.
UPDATE 2:32 pm EST: Looks like all the affected realms are back up early.