The DAT is offered year-round through the Prometric test centers. I recommend registering early – seats tend to fill up quickly and it’s common for the next available seat to be up to 2 months away!
Some students only study a few weeks, others like myself take up to 3 months. The study schedule on Bootcamp is ~8 weeks long, and I think that is a good length of time to study. You know yourself best and what it’ll take to do well on this test.
The majority of students study for the DAT while enrolled in classes, in the summer between junior and senior year. They start studying around April and take the test in the summer, around June through August. It's challenging, but it isn't anything harder than what you're going to do in dental school, consider it a test run!
I'd recommend taking an easier course load during the semester if possible, and starting studying around late March, planning for a June test date. That way you start studying while in school, then finish up finals, and can dedicate some time just to the DAT towards the end during the schedule. You can also reschedule the test to later in July if needed. It’s more important to take the DAT when you’re ready rather than early.
If you're really rusty on your basic sciences, you can go through the first half of the study schedule (the learning phase) on your own time, just to get a foundation back. Afterwards, you can restart the study schedule, and move through everything a little faster since you've already studied it somewhat, and it'll be more of a review instead of learning the material for the first time again.
Lastly, the DAT is the same difficulty throughout the year; there is no advantage to taking it during the summer or winter. Again, take the DAT when you’re ready.