They are currently booking through Feb. 3rd.
A few days ago, I was only getting Dings through Dec. 15th. Now I'm getting them through Feb. 3rd. So the answer is that sometimes they are available all the way to the end of SW's current schedule and sometimes they aren't. Even though I'm not going until March/April, I've been checking my Ding fares and when they've come up for my route (Portland, OR to Orlando), they've all been about the same as the normal Internet special fares. ($99-$120 one-way)