Well, after his conversion St Paul did say he spent three years in Arabia. And since he was the Apostle to the Gentiles you can bet they had three years of good preaching. Galatians 13-18
15 But when God, who set me apart from my mother’s womb and called me by his grace, was pleased 16 to reveal his Son in me so that I might preach him among the Gentiles, my immediate response was not to consult any human being. 17 I did not go up to Jerusalem to see those who were apostles before I was, but I went into Arabia. Later I returned to Damascus.
18 Then after three years, I went up to Jerusalem to get acquainted with Cephas[b] and stayed with him fifteen days.
There may have been converts as early as Pentecost:
Acts 2:5-11 NKJV
And there were dwelling in Jerusalem Jews, devout men, from every nation under heaven. And when this sound occurred, the multitude came together, and were confused, because everyone heard them speak in his own language. Then they were all amazed and marveled, saying to one another, “Look, are not all these who speak Galileans? And how is it that we hear, each in our own language in which we were born? Parthians and Medes and Elamites, those dwelling in Mesopotamia, Judea and Cappadocia, Pontus and Asia, Phrygia and Pamphylia, Egypt and the parts of Libya adjoining Cyrene, visitors from Rome, both Jews and proselytes, Cretans and Arabs—we hear them speaking in our own tongues the wonderful works of God.”