The Chasam Sofer (Even haEzer #119) writes that the amount of time that elapses before forgetfulness sets in is one year. Assuming the reading of parshas zachor from a Torah text is a din d’orasya, it therefore must be repeated once a year to fulfill the obligation of “zachor – lo tishkach”, not forgetting Amalek. How then do we fulfill the mitzvah in a leap year when more than 12 months have passed since the last reading of zachor? The Ch”S answers that the shiur is one year, not 12 months. During a leap year, the shiur extends by a month.
My impression is that this Ch”S is relatively well known, but less well known is the Sefer haChinuch who addresses this same issue. Pointing to the fact that the minhag in Eretz Yisrael was to lein the entire Torah over three years instead of one, the Sefer haChinuch writes that there is no obligation to hear zachor on an annual basis. How often must one hear the parsha? The Minchas Chinuch suggests that perhaps even hearing it once a lifetime would fulfill the obligation.