Based on Clement of Alexandria Mark wrote before Peter was martyred in A.D. 64-67. Thus Mark had to have been written before then. There is no evidence that eliminates a date as early as A.D. 40, however Mark was most likely written sometime in the early 50's or late forties.

Sources:
Encountering the New Testament, Walter A. Elwell / Robert W. Yarbrough, 1998, Baker Books, page 88

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 127-128