For dropdowns, you simply have to wait until they send you the mesaage they will accept your recruiting efforts
For pulldowns, you should wait until close to when signings start. If your targeted recruit is not considering a D2 school (or a high prestige D3 one) then you can start sending scouting visits. It takes anywhere from 10-20 scouting visits to get them to consider you, which is why pull downs are best for recruits who are closer to you