Generally Southwest has at least 105 days (3.5 months) open for booking. Once it gets down near that 105 day level, sometimes sooner, they'll open up new dates. So, really anywhere between 3 and 6 months will be available for booking at any given time.
Check out this thread: http://www.disboards.com/showthread.php?t=1057722It has the historical release dates for SW. I came across it in a similar thread a while ago and check it often for when I can start watching for good deals!