Watchtower 1970 p120
"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."
Watchtower 2015 1st June
"The Bible fixes no duration for the six creative “days.” Instead, it opens the door for modern scientists to study them and assign accurate time spans to them. We know that the creative “days” were much longer than 24-hour days."