Normally, how long would the hepatitis b vaccine stay in your body?

Good question. When someone gets vaccinated against hepatitis b, we can measure the level of antibodies produced. This gives us a sense of how protected people are. But, other kinds of immune cells (t cells) are also important in preventing chronic infection and we do not measure those. So protection probably lasts longer than ab do, but if you have an exposure, get tested for ab and boosted if levels are low.