Once you've defeated Deathbringer Saurfang in Icecrown Citadel, your faction's NPC will tell you how many days are left until the next gate is unlocked. The length of time is currently listed as 28 days, which means the Plagueworks should open on North American realms on Jan. 5, 2010.
Members of the Wowhead staff and our very own Fewyn decided to take a closer look at the timeline and estimate when we'll get the chance to battle the Lich King. If 28 days remains the standard length of time between the gate openings, the Frostwing Hall and the Frozen Throne should open up on March 2, 2010.
UPDATE: Daelo confirmed on the official forums that the 28-day wait for Plagueworks is simply due to the holidays. The delay between the launch of the rest of the wings will be less than 28 days, so the assumed March 2 date is late.
Keep in mind that this final date is purely speculation on the part of the Wowhead staff and is not an official announcement. What do you think about the possibility of a three-month wait? Is it a good length of time or too drawn out?