As a whole, airlines post schedules differently. Most legacy carriers (AA, DL, UA) post about 11 months in advance, but may change flight times or equipment closer to the summer. A few carriers post schedules about 6-months out.
If you're really focused on DL's schedule practices, let us know, and I'll move the thread to the DL forum for more discussion. Ocn Vw 1K, Moderator.