The kjv says "since you believed" and the Greek in the interlinear says "having believed" which in both cases is a different connotation than "when you believed" which implies it happens or happened immediately and upon belief only which it didn't. Note that in verse 6 of 19, even after they were baptized in the Lord's name,the Holy Ghost still didn't fall on them immediately, but it required the laying on of Paul's hands, an apostle, to impart the manifestation of the Spirit. Upon their baptism however, they would have received the gift of the Holy Ghost consistent with Acts 2:38, but not the miraculous manifestation of the Spirit, a gift or gifts of the Spirit, but just it's indwelling.