How often do you need small pox vaccination?

Never. Smallpox is a dead disease. It has been eradicated. Military personnel may be give smallpox vaccine because of the fear that some nation storing the virus may use it as a biologic weapon. Other than that, forget about it.
NEVER&MAY BE NEVER. Since 1973 we stopped using the Small Pox Vaccine in USA as this deaded Infectioius Disease has been eliminated from this earth with successful immunization So at this time end of 2015 Nobody in this world needs Small Pox Vaccine That is a fact.
You don't. The WHO announced World-wide eradication of Smallpox on May 8 1980. Smallpox has no animal reservoirs. So Smallpox would not return naturally. The American public has not received routine smallpox vaccinations since 1972. The government only provides smallpox to several hundred scientists/ medical personnel in research settings.