Here is the oration that the Church prays in the Mass and Office for Saint Peter of Alcántara (October 19):
Deus, qui beátum Petrum Confessórem tuum admirábilis pœniténtiæ et altíssimæ contemplatiónis múnere illustráre dignátus es: da nobis, quǽsumus; ut, eius suffragántibus méritis, carne mortificáti, facílius cæléstia capiámus.
Here is my translation:
O God, who hast deigned to render blessed Peter Thy Confessor illustrious for his gift of penance and highest contemplation: grant us, we beseech Thee, that for the sake of his favorable merits, by the mortification of the flesh we may more easily obtain heavenly things.
Here is the translation from the Divinum Officium site:
O God, Who hast been pleased to set before us in thy blessed Confessor Peter a wondrous example of penance and of a mind unfathomably rapt in thee, let, we beseech thee, the same thy servant pray for us, and him do Thou accept, that we may so die unto earthly things, as to take lively hold on heavenly things.
Reading even a brief account of the life of Saint Peter of Alcántara, such as the entry in Dom Prosper Guéranger’s Liturgical Year, is sufficient to tell us why special mention is made in this oration of his “gift of penance and highest contemplation.”
We all need to mortify the flesh, at the very least in a way to keep us free from mortal sin, but those who love God want to do more. I have been told that Franciscan Friars, even in very strict communities, are warned not to attempt the degree of severity that their illustrious brother, Fray Pedro de Alcántara, was so famous for. The logic that can be found in some of the works of Saint Alphonsus is, I believe, very helpful for us when considering this question of “how much penance” is for us. After showing several examples of extreme penitence, Saint Alphonsus tends to challenge us, not to copy the specific acts, but to say to ourselves: “If this saint could do so much, surely, I can do X.” Supernatural prudence, charity, and wise spiritual direction can fill in that blank for us.
Meanwhile, if we do nothing by way of penance over and above the very little the Church presently demands of us in Lent (never mind the presently non-binding ember days!), we are assured of two things: (1) a long purgatory, and (2) a very uneasy time of it when we read the life of Saint Peter of Alcántara!