honestly there is no specific time to study for the DAT. everyone is different when it comes to learning. Some are able to study for a month and get a 20/20/20 while others study 3months and can only pull an 18. I would start out by taking a practice test and see what your weak areas are in. Next, start studying your topics. Review everything once and then take another practice test to see where you are. If you are scoring well..go ahead and register for your exam and continue to review your notes and take practice exams. The best advice I can give you is make sure you know all biology (b/c the DAT is just that random) and really know your genchem concepts and organic reactions down cold. When I took the DAT i studied for about 2months everyday ~10hrs but not all 10 of those hrs were productive.
i wouldn't study for too long since you seem to forget things as time goes on, i would cram for about a month to 6 weeks max, then take the test. cramming = 8-10 hours a day. for I only had 3 weeks of real cramming, and got 20/21/21... but i think im going to be left high and dry this cycle since my app is super late.
It all depends on how you study and how much of the foundation you already know. I only had about 1.5 month to study...maybe 3-4 hrs a day because I was also working full-time. I was a bio major but had graduated over a year ago so all the basics I hadn't touched in 5 yrs so I had to re-learn it all before I could even start with mock tests. Ended up with 18/18/18...really wished I had put more time into studying but I did my best with what I had.