The event has been extended, and is now scheduled to end Friday, March 22nd. Portals will also spawn every half hour instead of every hour.
From March 11th to
Portals will appear in set locations in Valdrakken, Stormwind, and Orgrimmar roughly every hour on the hour. Hearthstone-themed bosses will spawn that have a chance to drop Sarge among other fun items.
Sarge is Bind On Pickup and not cageable, so you don't want to miss out on this event. Happy 10 years, Hearthstone!