Legacy carriers - Delta, American, United: 330 or 331 days in advance.
Southwest and jetBlue: periodically throughout the year; you can sign up for e-mail alerts, or check the websites (will generally provide advance notice of upcoming fare extensions), or spend time on the DIS Transportation forum, where helpful members typically post this information when they get it.
Spirit, Frontier, Allegiant: no idea, don't really care, but probably signing up for tgeir emails wiolbhelp.