According to Catholic Church, how long should a person pray for the soul of his/her dear ones after the death of the latter?

Catholics believe in the existence of Purgatory where one's soul gets purified before one is granted entry into the abode of God. So, we have November, the month dedicated to the prayer for the souls in Purgatory, as well as the special liturgical prayers for the dead . Now, one living person may go on praying for the soul of his/her beloved one who passed away , say before decades. In fact, we have no way of knowing whether the dead person's soul has attained heavenly abode, or not. For that matter, we do not even know the time, which is basically a human standard of truncating infinity, that one soul takes to purify itself in Purgatory. My question, therefore is: According to Catholic Church, how long should a person pray for the soul of his/her dear ones after the death of the latter ?