Two types are described: suddenly and step by step.
Summa Theologica, Part I of Second Part
Question 112. The cause of grace
Article 2. Whether any preparation and disposition for grace is required on man’s part?
Objection 2. Further, whoever is going on sinning, is not preparing himself to have grace. But to some who are going on sinning grace is given, as is clear in the case of Paul, who received grace whilst he was “breathing our threatenings and slaughter against the disciples of the Lord” (Acts 9:1). Hence no preparation for grace is required on man’s part.
Reply to Objection 2. Since a man cannot prepare himself for grace unless God prevent and move him to good, it is of no account whether anyone arrive at perfect preparation instantaneously, or step by step. For it is written (Sirach 11:23): “It is easy in the eyes of God on a sudden to make the poor man rich.” Now it sometimes happens that God moves a man to good, but not perfect good, and this preparation precedes grace. But He sometimes moves him suddenly and perfectly to good, and man receives grace suddenly, according to John 6:45: “Every one that hath heard of the Father, and hath learned, cometh to Me.” And thus it happened to Paul, since, suddenly when he was in the midst of sin, his heart was perfectly moved by God to hear, to learn, to come; and hence he received grace suddenly.