Study whenever you want but if I were you, I would work at NIH or some labs to get some experience and build relationships with professors (this might come in handy for LOR's). If you are a really disciplined sob, you should be able to fit in some DAT studying time in the evenings and nights.
I studied for the DAT during the spring break of my junior year after Gen Bio 1 & 2, Microbio, Genetics, Cell Bio, Gen Chem 1 & 2, Orgo 1 & 2, Biochem.
As you begin to take upper-level Bio and Chem classes, the stuff you had learned in Gen Bio's and other pre-req Bio and Chem clases should be tatoo'ed in your brain. By the time I finished Cell Bio & Biochem, I only had to look over the DAT review book just to refresh my memory.
By the time you open that blue Kaplan DAT book, everything you see should be just another review.
Good luck.