As Podil said, it depends on your sci background... and it also depends on your situation. I haven't studied anything science oriented in over 7 years.. Ochem and Bio in 9 years, GChem in 10 years. So it may take longer or shorter depending on your situation.
I would recommend using Kaplan Blue Book as your base of knowledge. If you feel comfortable that you know most of that material, there are other resources out there that you may want to check out...especially in Bio. Once you start taking practice exams and start doing practice problems, that will give you a more genuine assessment on where you stand.
I personally spent about 3 weeks getting reacquainted with the sciences, another couple of weeks doing practice problems, and the last month will be exams. In all, it will be a total of 2.5 months/about 5 hours a day. I never lost the QR, and PAT you'll acquire with practice. The last month of my studying is going to be practice exam intensive with occasional comprehensive reviews of the sciences using Kaplan BB. If I'm scoring well on practice exams, I'll stay on schedule and take the DAT in January. If not, we have the luxury of rescheduling if need be.
Good luck.