Acts 2:41 (NIV)
41 Those who accepted his message were baptized, and about three thousand were added to their number that day.
So, if eleven Apostles stood in the Jordan for 10 hours and baptized 1 person every 2.2 minutes then they could baptize all 3000 people in 10 hours. Can you imagine the workout dunking that many people!
It is probably better to assume that all 120 disciples who were in the upper room (Acts 1:14-15) did the baptizing of all the people. That would make it 24 minutes per baptism if they spent 10 hours which is probably too much time. That means that Jesus’ mother Mary and his brothers mentioned in Acts 1 did some baptizing that day. Probably among the 120 were also Mary Magdalene, Joanna, and Susana. They probably did some baptizing too. What a day that must have been!.