B.H. Roberts states that the revelation on plural marriage was received in 1831.
B.H. Roberts, ed., History of the Church of Jesus Christ of Latter-day Saints, 7 vols. (Salt Lake City: The Church of Jesus Christ of Latter-day Saints, 1909), 5:xxi-xxx
There is indisputable evidence that the revelation making known this marriage law was given to the Prophet as early as 1831. In that year, and thence intermittently up to 1833, the Prophet was engaged in a revision of the Bible text under the inspiration of God, Sidney Rigdon in the main acting as his scribe. As he began his revision with the Old Testament, he would be dealing with the age of the Patriarchs in 1831. He was doubtless struck with the favor in which the Lord held the several Bible Patriarchs of that period, notwithstanding they ha a plurality of wives. What more natural than that he should inquire of the Lord at that time, when his mind must have been impressed with the fact-Why, O Lord, didst Thou justify Thy servants, Abraham, Isaac and Jacob; as also Moses, David, and Solomon, in the matter of their having many wives and concubines (see opening paragraph of the Revelation)? In answer to that inquiry came the revelation, though not then committed to writing.
Corroborative evidences of the fact of the revelation having been given thus early in the Prophet's career are to be found in the early charges against the Church about its belief in "polygamy." For example: When the Book of Doctrine and Covenants was presented to the several quorums of the priesthood of the Church for acceptance in the general assembly of that body, the 17th of August, 1835, an article on "Marriage" was presented by W.W. Phelps, which for many years was published in the Doctrine and Covenants. It was not a revelation, nor was it presented as such to the general assembly of the priesthood. It was an article, however, that represented the views of the assembly on the subject of marriage at that time, unenlightened as they were by the revelation already given to the Prophet on the subject. What the Prophet Joseph's connection was with this article cannot be learned. Whether he approved it or not is uncertain, since h was absent form Kirtland at the time of the general assembly of the priesthood which accepted it, on a visit to the Saints in Michigan (see HISTORY OF THE CHURCH, Vol. I, pp. 243-52).