It really depends a lot on how prepared you are already. And how much effort you are willing to put in/ how perfectionist you are and what score you want.
I say this especially if it has been a while since you took the courses and you took them only once. Versus someone who just finished them. was a bio major who knows genetics, physiology and physical chem backwards and forwards.
The biggest issue I had were gaps in knowledge. For example, my chem class never went over phase changes/ solutions. So I had to learn it on my own and that sucked! I also had to learn the lytic/lysogenic cycles, the lac operon, etc. And some stuff on ethers, phenols and carbohydrates.
It really also depends how perfectionist you are. And how high/low your GPA is. I'd say for someone who is already well prepared and has all day to study, 6 weeks. For others it could be 6-7 months of on and off studying, perfecting your PAT skills, etc. Hope this helps.