Hey there, I found some info from the CDC that in a healthy person the shot can last up to 20 years.
I thought you might want to read this
http://www.cdc.gov/hepatitis/HBV/HBVfaq.htm#-
if you are exposed to hbv as an adult you clear it by immune system, hbv gets chronic at birth from the mother when there is no immune system.
only 2.8% of adults, probably with damaged immune system, can get the chronic infection
vaccine is not always for life and it fails in more than 15-20% cases.
it is very badly desinged, this is one of the many studies
http://www.ncbi.nlm.nih.gov/pubmed/25504950
but you can have it again after you test hbsag negative hbsab negative, the test is extremely simple/cheap and ready same day in god labs.after 20days from first shot recheck hbsab to see if you are a responder and if not recheck after 3 rd dose