This article is from the Childhood Vaccinations FAQ, by Lynn Gazis-Sax firstname.lastname@example.org with numerous contributions by others.
We don't know yet. It is estimated to last at least six
years. (Lancet, April 16, 1994) "Children immunized as long as six
years earlier continued to be well protected. . . . So far, the US
data show persistence of antibodies for three to four years after
immunization; data from Japan show persistence of antibodies for seven
to 10 years in healthy children." (Gershon)