Will the immune memory for high risk hpv disappear after about 20 years?

Usually much faster -- typically a couple of years. However, HPV DNA may persist, with a potential for reactivation many years later. But the exact frequency at various intervals isn't known (i.e. 5 vs 10, 20, or 30+ years).

A history of "high risk" strains of HPV should require annual pap smears, as HPV can resurface at any time. Slow acquisition of immune status has been reported , but it is unknown about how long sustained "immunity" may last.