I did the same thing, got the first two Hep B shots and then quit the series. A few years later I had my internist titer my antibodies and found I had a full immune response. She said if I hadn't, I would have had to begin the series over from the beginning, so I got lucky. I guess it varies from person to person. A few are good to go after two, almost everyone is done after three, and a few people need a fourth. But if you quit early and later fail the titer, you have to start over apparently. Good luck on your antibody count.