How long was the interval between the Ascension and Pentecost in Acts?
The interval was ten days. After the Lord ascended from the Mount of Olives and the disciples returned to Jerusalem, they gathered in an upper room with about 120 believers. They remained in that ten-day period until the Holy Spirit came down in Acts chapter 2.
Les explains that from the Ascension in Acts chapter 1 until the coming down of the Holy Spirit in Acts chapter 2, there is a ten-day interval. During those days the disciples were back in Jerusalem, gathered in an upper room. The group numbered about 120, likely including the eleven apostles plus other men, women, and probably children. Peter stood up among these believers in that ten-day period between the Ascension and Acts chapter 2.