Usually late during your HS junior year. My daughter applied online, summer of her junior year (going into senior year) and she received news of scholarship qualifications as soon a January of her senior year. everything was finalized in April. So for AROTC, check the cadet command website you can start your application right there. not sure if it is possible with nrotc, but probably.

My daughter applied for NROTC this past summer (going into her senior year) while completing her USNA application. She was notified of her scholarship to Purdue in early Oct.