Though there seems to be a growing trend for ridiculously early "commitments", most players are still making their final decisions in the spring of Grade 11 through to September--once they, and the coaches, know they are going to have the grades and scores required to secure admission. As you say, Ivies are often not until the fall of Gr 12 since the Admissions needs to be directly involved in the decision-making.
That means if some D1 schools are still not showing you much interest by the end of Gr 11 (or your scores are still too low), an offer is not likely forthcoming and you should be into heavy duty contingency planning: D3, CIS, or possibly a gap year in some cases.
This is a big part of the reason investing in SAT/ACT prep--early and often-- is so critical. There are far too many cases of players having interest from schools but unable to "pull the trigger" because they didn't have the scores and grades they needed and were forced to keep writing in the fall of Gr 12. That's generally far too late. It's incredible that parents will invest many thousands in off-ice training, skills development, and fancy programs and tournaments and think nothing of all the time their kids miss school for hockey. But they penny-pinch on the academic tutoring and SAT prep programs that end up being the thing that prevents them from accessing their schools of choice. Really dumb.
Often it's actually the so-called Blue Chip players most guilty of this. They tend to miss more school than most due to national camp commitments, and they or their parents seem especially inclined to believe that their hockey talent will trump their lack of scholastic talent/preparation. Often, teams may hold back a scholarship until late in Gr 12, hoping that somehow the scores will materialize or they can pressure admissions to make an exception. For this reason (as well as career-ending injuries), it is not unheard of for players (blue chip or otherwise) to get a D1 opportunity later in Gr 12.