How long had Paul been ministering to the Gentiles before he came back to Jerusalem?

Les explains that Paul had now been out ministering to Gentiles for some 20 years or more. He had been having a tremendous response, establishing churches throughout Asia Minor, Greece, and Rome. Then he comes back to Jerusalem, where he just about gets killed by the mob. He comes before the religious leaders in Jerusalem in Acts 22 and recounts his whole conversion experience on the road to Damascus. He also recounts his experiences among the Gentiles and how they were coming to a knowledge of Israel's God through the Gospel, through Christ's death, burial, and resurrection. Paul rehearses what happened years back: while praying in the temple, he was in a trance and saw the Lord say, "Make haste, get thee out of Jerusalem, for they the Jews will not receive thy testimony concerning me." Paul tells his Jewish audience that they know he imprisoned and beat in every synagogue those who believed, and when Stephen's blood was shed, he stood by consenting and held the raiment of those who killed him. Then the Lord said, "Depart, get out of Jerusalem, for I will send you far hence to the Gentiles."