Acts 8:35-39 Then Philip opened his mouth, and began at the same scripture, and preached unto him Jesus.
And as they went on their way, they came unto a certain water: and the eunuch said, See, here is water; what doth hinder me to be baptized?
And Philip said, If thou believest with all thine heart, thou mayest. And he answered and said, I believe that Jesus Christ is the Son of God.
And he commanded the chariot to stand still: and they went down both into the water, both Philip and the eunuch; and he baptized him.
And when they were come up out of the water, the Spirit of the Lord caught away Philip, that the eunuch saw him no more: and he went on
his way rejoicing.
This passage is a biblical model of salvation and baptism. Baptism should follow salvation, but there is not a prescribed amount of time that MUST pass between the two.