Half way through your junior year is a good time to get the ball rolling. Flight hours won’t make or break your kit, but do express interest in the field. Go do a discovery flight cost $100-$150 (almost all flight schools have them), I’d recommend going and doing one of those over your winter break. You get a feel for flying and they’ll let you attempt to land the plane! Now’s a good time to start working on LORs (also not required but helps build a stronger packet). I’d recommend now you go in and talk to a recruiter if your goal is to graduate and go to OCS. I started halfway through my junior year, I did not get into OCS until 1.5 years after graduating… sometimes it’s timing that plays a factor