It was partaken of when he was sick, but as his thanksgiving after Baptism showed, he was prepared to live out a full life after this
From the Life of Constantine by Eusebius of Caesarea:
CHAPTER LXI: His Sickness at Helenopolis, and Prayers respecting his Baptism.
AT first he experienced some slight bodily indisposition, which was soon followed by positive disease. In consequence of this he visited the hot baths of his own city; and thence proceeded to that which bore the name of his mother. Here he passed some time in the church of the martyrs, and offered up supplications and prayers to God. Being at length convinced that his life was drawing to a close, he felt the time was come at which he should seek purification from sins of his past career, firmly believing that whatever errors he had committed as a mortal man, his soul would be purified from them through the efficacy of the mystical words and the salutary waters of baptism. (1) Impressed with these thoughts, he poured forth his supplications and confessions to God, kneeling on the pavement in the church itself, in which he also now for the first time received the imposition of hands with prayer. (2) After this he proceeded as far as the suburbs of Nicomedia, and there, having summoned the bishops to meet him, addressed them in the following words.
CHAPTER LXII: Constantine's Appeal to the Bishops, requesting them to confer upon him the Rite of Baptism.
"THE time is arrived which I have long hoped for, with an earnest desire and prayer that I might obtain the salvation of God. The hour is come in which I too may have the blessing of that seal which confers immortality; the hour in which I may receive the seal of salvation. I had thought to do this in the waters of the river Jordan, wherein our Saviour, for our example, is recorded to have been baptized: but God, who knows what is expedient for us, is pleased that I should receive this blessing here. Be it so, then, without delay: (1) for should it be his will who is Lord of life and death, that my existence here should be prolonged, and should I be destined henceforth to associate with the people of God, and unite with them in prayer as a member of his. Church, I will prescribe to myself from this time such a course of life as befits his service." After he had thus spoken, the prelates performed the sacred ceremonies in the usual manner, and, having given him the necessary instructions, made him a partaker of the mystic ordinance. Thus was Constantine the first of all sovereigns who was regenerated and perfected in a church dedicated to the martyrs of Christ; thus gifted with the Divine seal of baptism, he rejoiced in spirit, was renewed, and filled with heavenly light: his soul was gladdened by reason of the fervency of his faith, and astonished at the manifestation of the power of God. At the conclusion of the ceremony he arrayed himself in shining imperial vestments, brilliant as the light, (2) and reclined on a couch of the purest white, refusing to clothe himself with the purple any more.
CHAPTER LXIII: How after his Baptism he rendered Thanks God.
HE then lifted his voice and poured forth a strain of thanksgiving to God; after which he added these words. "Now I know that I am truly blessed: now I feel assured that I am accounted worthy of immortality, and am made a partaker of Divine light." He further expressed his compassion for the unhappy condition of those who were strangers to such blessings as he enjoyed: and when the tribunes and generals of his army appeared in his presence with lamentations and tears at the prospect of their bereavement, and with prayers that his days might yet be prolonged, he assured them in reply that he was now in possession of true life; that none but himself could know the value of the blessings he had received; so that he was anxious rather to hasten than to defer his departure to God. He then proceeded to complete the needful arrangement of his affairs, bequeathing an annual donation to the Roman inhabitants of his imperial city; apportioning the inheritance of the empire, like a patrimonial estate, among his own children; in short, making every disposition according to his own pleasure. (1)
CHAPTER LXIV: Constantinople's Death at Noon on the Feast of Pentecost.
ALL these events occurred during a most important festival, I mean the august and holy solemnity of Pentecost, which is distinguished by a period of seven weeks, and sealed with that one day on which the holy Scriptures attest, the ascension of our common Saviour into heaven, and the descent of the Holy Spirit among men. In the course of this feast the emperor received the privileges I have described; and on the last day of all, which one might justly call the feast of feasts, he was removed about mid- day to the presence of his God, leaving his mortal remains to his fellow mortals, and carrying into fellowship with God that part of his being which was capable of understanding and loving him. (1) Such was the close of Constantine's mortal life. Let us now attend to the circumstances which followed this eventhttp://www.fordham.edu/halsall/basis/vita-constantine.html