I believe this depends on your own ability and how well understood/remember material from school. I had a 3.0 and did terrible in my classes so I started in May, studied for a solid 3 months everyday, doing between 8-12hrs/day. For the 1st two months I did 12hrs/day to get a hold of the material. the 3rd month was dedicated to practice tests and finding more detail to specific questions.
Because the test is getting harder or more random it will be best to have enough time to dedicate to a variety of study guides from Kaplan, Acheiver, Topscore, Cambells bio, Ap Cliff notes, Shuams bio, Destroyer, Exam Krackers, and many others.
Studied: May 5 - Aug 22
Exam: Aug. 22