I thought it ran through the end of the month, but I could be wrong. What I found out from the store manager in my area, however, is that they run crazy promotions all the time. AT&T isn't quite as worried about their pricing because they've got one hand in DirectTV now (and bundling it with their mobile customers). Verizon is in a similar situation. T-Mobile offers mobile only. He told me not to worry if I miss a promo because they run them like crazy to try and grab Verizon and AT&T customers.
I was looking for someone to get on the free third line and my brother came to mind (even though I'm married and out of the house). Just find a couple of very close friends or relatives (cousins, whatever) to jump into a deal like this with. My experience was that they're super willing to work with you to make you switch so I wouldn't fret about the current deals expiring too much.
Having said that, if you pay 70$ for the one plan (for one user) and get the 20% off promo for people switching through the end of March, you're still only looking at a 54$ bill monthly for everything unlimited. Might be worth considering for the time being (depending on what you're paying with your family). God, I was paying over $100 for my wife and I with 3 GB of SHARED data!