Joe,
This may sound a little trite, but you should apply when you have your best possible application package. Score as high as possible on the ASTB, perform as well as possible in the PRT, etc... I did pretty well on my ASTB but I asked my recruiter to let me take it again so that I could do better (he didn't let me). If this is something you really want, go the extra mile in every way possible and make your own luck.
You were probably wondering what time of the year is the best to apply. Most people would agree that there are more competitive and less competitive times to apply, but I don't think that it's the same few months each year. There are a lot of factors which determine how and when the slots open up. Hope that answered your question. Best of luck to you,
-Greg