There is no timetable on sending a card. If you feel like doing it, then you should go ahead. I think it is great you want to so soon and feel secure enough to do it. There also is no rule on what the therapist does with the cards. They probably each do their own thing, and I would assume many throw them out. What is important is not that they keep your card but that they read it and learn of your feelings/intent/gratitude -- whatever you are trying to convey. Everyone likes to feel appreciated, and I think a card from a client to therapist is a way to show appreciation.