As soon as you have believed! Don't delay.

"Those who believed...were baptized... that day !"
Acts 2:41

"Then Philip began with the scripture and told him the Good News about Jesus. As they traveled along the road, they came to some water, and the man said, 'Look, here is water! Why shouldn't I be baptized right now?' Philip said, 'If you believe with all your heart, you may.' The man answered, 'I believe that Jesus Christ is the Son of God.' So they went down into the water and Philip baptized him."
Acts 8:35-38