Paul wrote this letter approximate a year after 1 Corinthians (inferred from 2 Corinthians 12:14, 13:1-2). It most likely was written after a visit to Corinth. Paul was now in Macedonia, which is to the north of Ephesus. The date this letter was written is approximately A.D. 55.

Sources:
Encountering the New Testament, Walter A. Elwell / Robert W. Yarbrough, 1998, Baker Books, page 293

A Survey of the New Testament, Third Edition, Robert H. Gundry, 1994, A Survey of the New Testament, Third Edition, Robert H. Gundry, 1994, Zondervan, page 370