During the late 60's and 70's studies were conducted for 6 months, if student not attending some meetings, it was terminated.
Since then it has varied between one and two books that were to be studied.
*** km 7/04 p. 1 par. 3 Part 1—Conducting Progressive Bible Studies ***
The Require brochure and the Knowledge book are publications used for conducting many Bible studies. After they have been completed, if it is evident that a person is making progress, even slowly, and that he is developing appreciation for what he is learning, then the study may be continued in the WorshipGod book.
*** km 5/96 p. 7 Question Box ***
Our Kingdom Ministry for September 1993 recommended that a home Bible study be continued with newly interested ones until two books have been studied. Now that we have the Knowledge book, it seems advisable to make an adjustment in this procedure, as outlined on pages 13 and 14 of the January 15, 1996, issue of The Watchtower.
The Knowledge book is designed to help those "rightly disposed for everlasting life" to learn what they need to know in order to make a dedication to Jehovah and get baptized. (Acts 13:48) Therefore, after completing this new publication, it should not be necessary to study a second book with the same student. As your Bible students begin to embrace the truth, you can progressively encourage them to round out their knowledge by attending meetings of Jehovah’s Witnesses as well as by reading the Bible and various Christian publications
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But there was something more to the surprise that was in store for assembly delegates. The new Truth book was to be used in a six-month Bible study program. Because of the way this publication involved the student, generally by the time he completed it he would take some action, either for or against the truth. No longer would one of Jehovah’s witnesses conduct Bible studies with an individual year after year without the student’s making definite spiritual progress, acting upon the knowledge gained.
Actually you could study as long as you wanted. It was dependent on individual elders in each congregation who decided that studies should be cancelled if you wanted to turn in a slip.