Whether purgatory is instantaneous, or is experienced in time as we here on Earth understand time, or is experienced in a manner completely different from "time" is ... not a matter of doctrine. And the "number of days" indulgence, formerly used but now replaced, referenced a "number of days" of public, Earthly pennance. It did NOT reference "time" (again, it is not taught definitively whether souls in Purgatory experience "time" in a way we would understand it) in Purgatory, although many misunderstood it that way.
The whole notion is anti-biblical nonsense; folks die and go to heaven or hell until the final judgement when all are judged.