NROTC - Navy Option -- Per old thread on this site, in 2009, the NROTC Board met in August, 2009, then every other week or so through March, 2010. In the following application year, the expected August 2010 first Board did not meet, also per posts here, but held its first Board in September.
Army ROTC -- the past few years has met three times, starting in, I think, Sept/Oct, then early January, then early March.
Marine Option NROTC -- is more like Army... can't recall detail but meets only 2-3 times, beginning I think in November with first notifications sent out just before Christmas.
don't know about Air Force ROTC.
Bottom line -- the earlier you are able to get your file complete, the more times you will go before a selection Board. I have read that you can update SAT or ACT scores after your file is complete, between Boards, but cannot confirm that. If that is true, then certainly earlier is better than later as far as completing the file... Perhaps you can target June 1.