I believe that you gave make reservations for Dive Quest one year in advance. It all depends on the time of year you are going when you need to make your own reservations, because during the holidays it can be a little crazy. They will require payment when you make your reservation so if you can pay for it, make your reservations as soon as you can. You will have a better chance to get exact day and time that fits your schedule.
I just did Divequest on my last trip in March. I made my reservation for Friday March 25 while sitting at Cosmic Ray's on Wednesday March 23. I would have to say it just depends on the time of year that you want to go.
One thing about this tour is that so few are actually qualified. Although I made my original res last fall, I changed the time last week with no problem. July 13, second session, wave if you're there.