You've probably seen this before, right Hotdogs? This is the most recent mention I can find of the days being even in length:
*** w70 2/15 p. 120 The Days of Creation from God’s Viewpoint ***
Just how long, then, were these “days” of creation? The Bible gives us a clue as to the length of the seventh day. Since these “days” were all part of one ‘week,’ it would be reasonable to conclude that all these “days” were of the same length.
[...]This accounts for 6,000 years. Is that the length of the seventh day? No, because we read that “God proceeded to bless the seventh day and make it sacred.” Its outcome must be “very good,” and that is not true of present world conditions; so the “day” must still be continuing. Actually these six thousand years have been, as it were, man’s workweek, in which he labored by the sweat of his face. But he will get rest during the coming thousand-year reign of Christ, which Bible chronology and fulfillment of Bible prophecy show is to begin very soon.—Gen. 2:3.
The seventh one thousand years of the seventh “day” will thus in itself be a sabbath. During it Satan and his demons will be bound. Christ and his anointed followers will rule with him as kings and priests. With what result? That all God’s enemies will be put beneath Christ’s feet. By means of this sabbath the seventh day will truly be sacred, for it will cause righteousness to flourish.—1 Cor. 15:24-28; Rev. 20:1-6; Psalm 72.
Thus we find the seventh “day” of the creative week to be seven thousand years long. On the basis of the length of the seventh “day” it is therefore reasonable to conclude that each of the other six “days” also was a period of 7,000 years. This length of time would be ample for all that the Bible tells us took place on each of the six days of creation.
This passage also touches on the circular reasoning surrounding 7,000 days, which seems to be "Well, 6,000 years have passed so far, and the Thousand Year Reign must occur within this final day, so that makes 7,000", with no consideration of the possibility that maybe the day is 10,000 or 1,000,000 years long instead.