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.:

• Persons who have written documentation of a complete HepB vaccine series and who did not receive postvaccination testing should receive a single vaccine booster dose
• Persons who are in the process of being vaccinated but who have not completed the vaccine series should receive the appropriate dose of HepB immune globulin (HBIG) and should complete the vaccine series
• Unvaccinated persons should receive both HBIG and the vaccine as soon as possible after exposure (preferably within 24 hours). The vaccine may be administered simultaneously with HBIG in a separate injection site. The vaccine series should be completed in accordance with the age-appropriate vaccine dose and schedule

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.:

• Persons with written documentation of a complete HepB vaccine series require no further treatment.
• Persons who are not fully vaccinated should complete the vaccine series.
• Unvaccinated persons should receive the HepB vaccine series with the first dose administered as soon as possible after exposure, preferably within 24 hours. The vaccine series should be completed in accordance with the age-appropriate dose and schedule.

From http://www.cdc.gov/hepatitis/HBV/HBVfaq.htm:

HBsAg will be detected in an infected person’s blood an average of 4 weeks (range: 1–9 weeks) after exposure to the virus.