I took the DAT the summer after my sophomore year, and it was one of the best decisions I made in the whole process. I had credit from AP bio my senior year + intro bio II, cell bio, gen and organic chem done by that point. I focused on studying all summer, with some shadowing, and got it done in mid-August. Scores are good for 2-3 years, depending on the school. So if you plan to apply right after school, your scores are definitely fresh. If you don't like your scores, you could always study next summer as well. But definitely don't plan on retaking them. Study hard the first time and get it done! After getting it done, I was basically free to focus on my application and all of that good stuff the next summer.