James was martyred in A.D. 62, and thus this letter had to have been written prior to then. It may have been written as early as the late 30's. However, some of the internal evidence, such as the references to "diaspora" tend to favor a later date rather than a earlier date.

Sources:
Encountering the New Testament, Walter A. Elwell / Robert W. Yarbrough, 1998, Baker Books, page 354

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 432