From Insight Vol 1 p. 545:
Length of Creative Days. The Bible does not specify the length of each of the creative periods. Yet all six of them have ended, it being said with respect to the sixth day (as in the case of each of the preceding five days): “And there came to be evening and there came to be morning, a sixth day.” (Ge 1:31) However, this statement is not made regarding the seventh day, on which God proceeded to rest, indicating that it continued. (Ge 2:1-3) Also, more than 4,000 years after the seventh day, or God’s rest day, commenced, Paul indicated that it was still in progress. At Hebrews 4:1-11 he referred to the earlier words of David (Ps 95:7, 8, 11) and to Genesis 2:2 and urged: “Let us therefore do our utmost to enter into that rest.” By the apostle’s time, the seventh day had been continuing for thousands of years and had not yet ended. The Thousand Year Reign of Jesus Christ, who is Scripturally identified as “Lord of the sabbath” (Mt 12:8), is evidently part of the great sabbath, God’s rest day. (Re 20:1-6) This would indicate the passing of thousands of years from the commencement of God’s rest day to its end. The week of days set forth at Genesis 1:3 to 2:3, the last of which is a sabbath, seems to parallel the week into which the Israelites divided their time, observing a sabbath on the seventh day thereof, in keeping with the divine will. (Ex 20:8-11) And, since the seventh day has been continuing for thousands of years, it may reasonably be concluded that each of the six creative periods, or days, was at least thousands of years in length.
I recently read something similar, but can't recall where, but it basically implied that each day was of varying length. About the only thing they stick to is that Adam, the first human, was created a little over 6,000 years ago.