Mostly depends on your background.
I studied about 3 weeks for the test. My bio was weak, so I spent the majority of my time on that section...learning about plants, jellyfish, ecology, etc... crap that I hadnt had since high school. However, other sections like math, reading comp, and orgo, I was able to get by with spending about 5 hours total on those sections. So, if your strengths can knock off a couple of the sections, then its very possible to get an 18 with 2 weeks of studying.