In Acts 14:21 it says that Paul and Barnabas preached the good news in the city of Derbe and made many disciples.
These guys were not in Derbe very long (just a few days up to a few months) and it tells us they made many disciples. So how long does it take to make a disciple? Does this passage make you wonder that, under the right circumstances, someone can be discipled in a few days?
A disciple is a follower of Jesus Christ. How long does it take to help someone follow Jesus? I think the church often complicates a very simple process of helping someone give their life over to Jesus and follow his teachings. The church has owned the spiritual development of Christ followers, but all Jesus asked us to do is make disciples, make a follower of Jesus. The responsibility of becoming like Jesus is primarily in the hands of the follower. The church preaches and teaches, but the follower chooses to live it out in relationship with Jesus and his church.