I took my DAT in my 3rd year - if you're planning on entering d-school after you've completed your bachelors, then this is a good time to take the DAT. Your marks will be recent enough for any school, and if you want to re-take the DAT, then you'll have plenty of time.
As for studying for the DAT, I started really studying for the DAT about a month or two before my test. It's hard to gauge how much time one would require, because the Canadian DAT is only written in November and February, so you're always studying for it while you're in school. So it might take you less time to feel prepared for the DAT than I did.
To study, I enrolled in the Kaplan online prep course... I didn't find the lectures to be very helpful, but the prep book was nice to have as it summarized a lot of the information that is testable. Other prep materials that I have heard are useful are Schaum's Biology and some DAT practice tests that are online (?)... I only used the ones provided by Kaplan
