The Handbook of Indulgences states, "A partial indulgence is granted the Christian faithful who read sacred Scripture with the veneration due God’s word and as a form of spiritual reading. The indulgence will be a plenary one when such reading is for at least one-half hour" (80).
Keep in mind that there are several requirements for acquiring a plenary indulgence. The Apostolic Constitution on Indulgences explains,
To acquire a plenary indulgence it is necessary to perform the work to which the indulgence is attached and to fulfill three conditions: sacramental confession, Eucharistic Communion and prayer for the intentions of the Supreme Pontiff. It is further required that all attachment to sin, even to venial sin, be absent. If this disposition is in any way less than complete, or if the prescribed three conditions are not fulfilled, the indulgence will be only partial. . . .