How long did Paul train before he began his ministry?

There were at least 2 elements of Paul’s training.

1. His training at the feet of Gamaliel (Acts 22:3) according to the strict manner of the law of the Jews.
2. The revelation of the Gospel directly from Jesus Christ (Gal 1:12), most probably in Arabia.

Probably his training with Gamaliel would have been concluded by the time he was twenty and would have given Paul a very good understanding of Old Testament Scripture. The school of Hillel (which Gamaliel’s family belonged to) concentrated on the Scripture in the education process.

His training in Christianity was directly from Jesus. We have no idea how long this would have been, but we can make some guesses.

It was probably in Arabia that this revelation was given to Paul, as Paul went there early on from Damascus – he did not immediately consult with anyone (Gal 1:16). He then returned to Damascus for 3 years (Gal 1:18), before going to Jerusalem. This must have occurred before AD 40, as we learn from 2 Cor 11:32 of King Aretas’ involvement and Aretas died in AD40. So if we take the 3 years he was preaching in Damascus from AD40, we get AD37. So we are approximately 4 or 5 years after the crucifixion. Saul’s persecution of the Christians would have taken up some time, so we are not seeing an extended time period available for Paul’s training.