I think they've flip-flopped (or, hit the "new light" -switch on and off and on again) a few times on this, because in C. T. Russel's day, he said in the Photodrama of Creation that it was 7000 years each. But then I think later on, in the fifties or so, the understanding was that it was a thousand years pr. creation day. I'm not sure if they have ever came out and said "from now on it's an indefinite period of time", but they never do it that way; it's just suddenly snuck in there in a sentence, plus the earlier teaching is not mentioned anymore in articles you'd think it wold be mentioned. Then again, it should be possible to find the approximate time when they went from one teaching to the other.
[edit] I've checked, and it was "thousands of years" pr. day in the 1985 Creation book. It would make sense that the 7000 years (or 1000) pr. day teaching would have been in effect until around 1975, but not much longer. [/edit]