The timeline will vary based on talent. The top shelf, jaw dropping, potential top ten round draft choices will be recruited whenever they show their potential. For some it could be as early as freshman year. For a majority of them it will be post soph year.The rest of the D1 prospects will be post soph or post junior year.
It's a funnel of prospects. As kids realize they aren't D1 prospects or the right D1 isn't interested in them they start looking at the D2s and D3s. For academics D3s the timeline may come down to Early Decision in the fall of senior year or even the following April.
A friend of my son wasn't quite the talent for a top academic D3 coach to use one of his recruiting chips with admissions. The kid was wait listed academically. He didn't know where he was playing college ball until May when he was accepted.