When he says that he was called by his grace, it seems to point to his call on the Damascus Road around 34AD in Acts 9. And when he says that God had revealed his Son in me, that I might preach him among the heathen, it seems to be referring to the revelation of the mystery that occurred some later. So, the text is simply saying that he was separated at birth, called on the road to Damascus, and received the revelation somewhere between Acts 9 and 13.
He also mentions that upon receiving the revelation of the mystery, that he did not confer with flesh and blood. The point is that he did not have to because they would not have known what he was talking about because it was given uniquely to him.
Verses 17-19: Neither went I up to Jerusalem to them which were apostles before me; but I went into Arabia, and returned again unto Damascus. Then after three years I went up to Jerusalem to see Peter, and abode with him fifteen days. But other of the apostles saw I none, save James the Lord's brother. He also makes it clear that he did not visit the Twelve in Jerusalem either regarding to the revelation that he had received, but instead, he went to Arabia and then to Damascus for a period of three years. This period is covered in Acts 9:19 when Paul returned to Damascus. He later did go to Jerusalem for fifteen days where he visited with Peter and saw James; therefore, he cannot be referring the council of Acts 15.
Notice that verse 19 refers to James, the Lord's brother, as an apostle. He would have been one of those selected by men that Paul referred to back in verse 1 since Jesus did not appoint him during his earthly ministry.
Verse 20: Now the things which I write unto you, behold, before God, I lie not. Why would Paul even say this? - Because apparently people were accusing him of being a liar regarding the revelation that he had received. The question that I have is when was the revelation given to him? Like I said, he was converted on the Damascus Road in 34AD and he apparently received the revelation sometime afterward. To me, it could not have been during the three years he was in Arabia or he would have mentioned it to Peter when he saw in in Jerusalem during his fifteen day stay. Again, to me at least, it had to have been sometime after the visit and before the Council in 52AD. That is a period of nineteen years. However, the verses here in Galatians seem to indicate that he received it prior to the visit with Peter (cf. verses 16-18).
In my opinion, Paul is merely looking back as he is writing. In other words, he is not giving a precise time lime at all. He is just saying that he was to receive the mystery to preach to the heaven. However, instead of getting lost in the weeds here, Paul's point is that the mystery did not come from Peter.
Again, I believe that he received it during the fourteen years before he returned to Jerusalem for the council in 52AD as he mentioned in Galatians 2:1-9. Also, 2 Corinthians 12:7 seems to place this date around 46AD since that he when he wrote it.
Also, another time indicator is that by the time he arrived at the Council in Acts 15, he mentions that there are churches that were made up of Gentiles in Syria, Silicia, and Antioch which is where he went after leaving Jerusalem. Again, a lot of conjecture there on my part, but compelling.