there isnt a set no. of pages that the journal should be but u should aim to make it as comprehensive and detailed as u can. It can contain entries dating back to every single little thing u did that was related to the project. My teacher encouraged us to even include entries detailing trivial things we did such as browsing the internet for information, conducting historical readings or even discussing the question with people, In the end mine contained some 25 entries and went for around 7 A4 pages, but it still got full marks. Of course the marking regimen will probably vary from school to school but there is no definite, set no. of pages required