Under the pontificate of His Holiness, Pope St. Pius X, the Sacred Congregation of Indulgences issued an answer to this very question, as found in the Raccolta. This version is from 1910.
116. Ejaculation.i. Seven Years and Seven Quarantines.* T.Q. ii. Plenary, once a week, if used daily, II.* (See Instructions.)116 Pius X, May 18, 1907.DOMINUS meus, et DEUS meus. My LORD and my GOD.*These words are to be said with faith, piety, and love, while looking upon the Blessed Sacrament, either during the Elevation in the Mass, or when exposed on the altar.
Consider that, at the time, indulgences were counted in days and years. This can translate to current norms of partial and plenary, if that helps.
Interestingly, a particular missal instructs the reader to say those very words at the Elevation of the Sacred Body, and then suggests the following for the Elevation of the Precious Blood.
Be mindful, O Lord, of thy creature, whom Thou hast redeemed by Thy Most Precious Blood.It would be wonderful to teach children this practice. If the belief and understanding of the teaching on the Real Presence is truly reflected in these recent, dire, poll results, then simple catechism like this can begin to effect change. And if all we can accomplish is some personal time off purgatory, then that's probably good enough. We'll take it!