It's really hard to gauge what material you will be tested on during the BIO section. Most will refer you to Cliff's AP Bio or Barron's AP Bio and tell you to memorize that. Gives you detailed summary and the amount of time it takes you to cover Campbell's once is about as much time you can cover either of those books at LEAST two-three times.
Plus, it's better that you just focus your Natural Science studies on areas that could easily net you above 20. GC and OC supposedly are easier to score above a 20 compared to BIO.
I would only use Campbell's as a reference if you're confused on a topic.
Good luck!