Whether or not the Creative days are 7000 years each or just 24 hours has nothing to do with the specific reason the WTS came to that conclusion. I just wanted to state that first.
Having noted the above, the basis for the WTS' belief (and mind also) that each creative day is 7000 years long is because it is believed that the millennium is a special sabbath of 1000 years. If the millennium represents a sabbath and thus the seventh division of the Creative Day, then it is presumed then that each Creative Day must be 7000 years each. This comes out of the interpretation of a reference in Hebrews where Paul as well suggests a rest day within a rest day, a sabbath within a sabbath:
3 For we who have exercised faith do enter into the rest, just as he has said: “So I swore in my anger, ‘They shall not enter into my rest,’” although his works were finished from the founding of the world. 4 For in one place he has said of the seventh day as follows: “And God rested on the seventh day from all his works,” 5 and again in this place: “They shall not enter into my rest.”
6 Since, therefore, it remains for some to enter into it, and those to whom the good news was first declared did not enter in because of disobedience, 7 he again marks off a certain day
In other words, because the Jews who had seen the great works of God at the time of the ten plagues soon rebelled, God promised none of those of that generation would enter his "rest", meaning that none of these would be selected for the first resurrection, which is the only way anyone who had died previously to experience the millennial reign of Christ. They would be resurrected later, of course, with the rest of mankind at the second resurrection where both the righteous and unrighteous are raised. The first resurrection are only for the anointed "144,000" (even though it is really 1,440,000, 144K is just 1/10th made up of the natural Jews). That's what this is about, qualifying for that "better resurrection" which allows the faithful to enter into the sabbath rest of Christ's 1000-year reign.
But as noted, Paul contrasts this with the Creative Rest Day, establishing an upcoming rest day within the 7th Creative Day. Based upon that, it is presumed by the witnesses and many others, actually, that at the very least, Armageddon must arrive before the last 1000 years of the 7th Creative Day. That is what 1975 was all about. They figured 1975 was 6001 years after creation. They gave Adam just one year to name all the animals and then get saddled with a wife the next year, so that the 6th Creative Day ended just 1 year after Adam's creation. In that case, based upon their chronology going back to Adam, 1975 marked the end of 6000 years, and since the millennium takes up 1000 years, they were expecting Armageddon to come at least by 1975. IT DID! For the WTS and their wrong prophecy, but not what they expected.
At any rate, that's where the specific concept of the 7000-year long Creative Days comes from. If you Google "Creative Day and 7000 years" you can see who else arrived at this same conclusion.
So the 7000 years IS Bible based, though the exact applications as far as chronology vary depending on the various chronologies.
JCanon