How long you study depends on how much time you devote to reviewing topics, and how long you spend thinking about the associated questions. On average, our best-performing members complete 3-5 questions per minute. Some of our highest-performing members have even condensed their entire study schedule into a short timeframe of just a few months: http://blog.firecracker.me/students/how-this-crunch-time-firecracker-scored-a-274-on-usmle-step-1
Pacing tips: To maximize your efficiency, I recommend checking out our keyboard shortcuts so you won't have to click your mouse at all during quizzing (Hit the "H" key on your keyboard during a quiz to discover the keyboard shortcuts we have available).
You certainly don't have to know every detail for each question; Firecracker's adaptive question scheduling algorithm will ensure that you're prompted to review any questions that you rate lower (1s and 2s) frequently: http://blog.firecracker.me/students/firecrackers-updated-algorithm
Make sure you're scoring your recall (1-5) according to the specific answer to each specific question; don't expect yourself to be able to memorize our entire topic summary cards, or even the additional context that expands when you click the "More details..." link. This being said, there's no need to rush through your questions. You'll get the most returns on your time if you're thinking critically and engaging your brain to recall the answer as best you can before choosing to Show Answer in Firecracker. If you don't get through all of your recommended "To-Do" questions in a day, that's nothing to worry about. Any questions you don't get to will be recommended for you to review soon on a future date.
Last tip I want to mention for increasing your quizzing efficiency; there's no need to even open up the "More Details..." or "Show related topic details" links for the majority of quiz questions you see. If you're doing all of the questions for a topic, then you'll effectively review the entire topic just by looking at the specific answers for that topic's questions. I'd suggest in general only hitting the "More Details..." link at the bottom of the answer when you're planning to rate that question a 1.
I'd also recommend you download the free Firecracker Daily Review native mobile app, which should make doing questions in Firecracker that much more convenient!