I think 4 semesters would be a good goal for completion of the pre-requesite courses. You could, of course, complete a whole year of chemsitry, biology, or physics during the summer, but it might be better to carry heavier courseloads during the fall and spring semesters to prove you can handle it. Do you plan to continue working as an RDH while you complete your pre-reqs? The decision to work will likely make a big difference in your timeline.
The chemistries have to be taken 1 at a time and there are 4-5. (5 includes biochem which is a pre-req for more and more dental schools every year).
-Gen Chem 1
-Gen Chem 2
-Organic 1
-Organic 2
-Biochem 1
For Biology you technically only need 2, the General Bios 1 and 2, which equals one year. However, it would be a good idea for your application and for the DAT to continue with more Bio courses, esp:
-Physiology
-Cell Bio
-Genetics
- Anatomy (not the pre-DH anatomy)
-Microbiology
-Immunology
Then there's physics, just take 1 year's worth of physics for non majors.
*Edit. I think I should add that there are math and English requirements too, but I kind of assumed that you already had those. I think they are pretty basic (Eng 101, Statisitcs etc). Look into it, they vary a little from school to school.
The DAT will contain Gen/Organic Chem and General Bio so you really can't take it until you at least finish those. Biochem and upper level Bio courses will definitely help you in the Bio section of the DAT too.For example, I can remember one specific question in the Bio section of the DAT that I never would have gotten right if I hadn't taken Comparitive Anatomy. I would set aside 7 or 8 weeks to study for it if you can! Check out the DAT forums on this site for study plans etc.
It might sound like a long journey, but it can be a very rewarding one! Good Luck!