I have taken the content from http://www.cdc.gov/mmwr/preview/mmwrhtml/00033455.htm and modified the titles a bit. I hope the CDC won't mind.
For people who definitely have been exposed to HepB via sex, needle-stick injury, etc.:
For people who have been in risky situations but are not sure if they have been exposed to HepB via sex, needle-stick injury, etc.:
HBsAg will be detected in an infected person’s blood an average of 4 weeks (range: 1–9 weeks) after exposure to the virus.