The photo-drama of creation stated "The Genesis account tells nothing about the formation of the Earth itself. It says, "In the beginning the Earth was--without form and void"--shapeless, empty. There were neither mountains nor valleys, trees nor shrubs, rivers nor oceans, but the Earth was. How long before that it had been created, is not stated." (Photo Drama of Creation). In all the JW literature I have seen, it has been stated that the earth existed for unknown periods of time before the first day.
From The Photo Drama of Creation up until the mid-80s, JW publications referred to the creative days as 7000 years long.
However, the 1985 Creation book stated "It would seem reaonable that the 'days' of Genesis could likewise have embraced long periods of time - milleniums" (Life How Did it Get Here - By Evolution or Creation, Chapter 3, pg. 27, par 7).
In 1987 they briefly went back to the 7000 years teaching in a Questions For Readers article: "a study of the fulfillment of Bible prophecy and of our location in the stream of time strongly indicate that each of the creative days (Genesis, chapter 1) is 7,000 years long." (Watchtower 1987 January 1 pg. 30).
After that articles have referred to the creative days as "thousands of years" (Awake! 2002 June 8 pg 10) or "considerable lengths of time" (Awake! 2006 September pg 19)