It was 3 weeks to the day from when the USCIS tracker changed. Online tracker changed 3/17, got the email notification that the tracker had changed 3/18, and the interview was on 4/7, card in hand 4/14. We got lucky and had the interview the day after our two yr anniversary so also got the 10yr card - definitely the only silver lining of waiting so long! Once the interview is scheduled, everything happened so fast; you're really in the last month/end game at that point.