It depends on what your science background is like. I studied for about two months because I remembered NOTHING from genchem. Also, if you wanna have the possibility (emphasis on possibility) of doing awesome in Bio, you need to leave no stone unturned. That means going through the bio in every freaking resource. Most people don't because even if you do this, there's no guarantees. If your time is limited, focus on getting a good bio score and ace the chemistry sections not because they're easier, but because they're more predictable.