DL allows bookings to be made 331 days in advance. So the schedules for flights will be loaded 331 days in advance, along with award inventory. As long as you're looking for DL metal flights, award inventory should be available on any flight that still has seats available (what level of inventory will vary based on demand for that flight). If you're looking for partner flights, then it can happen whenever. You won't know when it's about to released to DL and may only know if you use a tool like Expert Flyer to assist.

My personal experience: the schedule loading begins at midnight ATL time. Sometimes it takes a while to finish (maybe 1 hour or so). Being 3 hours behind ATL on Pacific Time, for whatever reason, I also had to adjust the time/time zone on my computer when I was making an award booking 331 days in advance. I was trying to book it around 10 PM Pacific Time, which was still technically 332 days away - it was 1 AM in ATL - 331 days out. DL.com kept erroring out. I adjusted the clock on my computer to Eastern time, thus moving the date ahead and to 331 days out and then it all worked. I don't know if it was a coincidence or what (and I did this a couple days in a row as I was playing with inventory and options so it was not one isolated incident).