If you're going to study over the summer 2 solid months will be plenty.
If you're going to study for the DAT while you are still in school, 3 months minimum because I doubt you'll be able to spare more than a couple hours a day. (Not saying anything about your work ethic...just that pre-reqs tend to be fairly rigorous courses)
Once you're done with O-chem you can take the DAT at any point (you can take it before, but you'll have to teach yourself an awful lot of O-chem!). O chem implies that you've already done Bio 101 types and gen chem so this would be the best time. You would be prepared for all the parts of the DAT's science section.