Great question, I think for some this is the beginning of interview preparation and it's good to know how to organize yourself in order to be in a good position during interview season. I'll answer these questions by drawing on my experience as well as that of other people I know who had success with their interviews.
1) I can't remember a specific number of days or weeks, it was more about practicing whenever I had a chance to work on cases or behaviorals. For people who are working in internships or full time, it's about making time (either after work or during the weekends). The key takeaway from here is that it really isn't about having a minimum number of hours you need to work, but rather finding a routine, staying consistent, and making the most out of your time (work smart and hard). A thing to note on behavioral interview questions: you can prepare for these on your own at first (which means more flexibility) before reviewing them with friends.
2) I would recommend preparing before you get an interview offer (ideally before you even apply). The more time you give yourself, the better you will be able to identify your shortcomings and work on them. Sure, some people have pulled off preparing after they get an interview invitation, but it hasn't been too frequent and it usually involves a lot of stress. This builds off point number 1: the sooner you start preparing the better your performance will be.
3) The application itself (per firm) can take anywhere from 2 weeks to 1.5 months (this usually depends on the office, the number of applicants, etc. and includes the time you'll wait to hear back between each round). I've seen cases where the application takes a week but this was a specific case where a firm had their first and second round interviews in subsequent days.
I think the best way to make use of your time is by creating a schedule where you practice with others and then can dedicate additional time to doing some self-studying in order to get your mind in "interview-mode". Because each interview round will last 2-5 hours (depending on the firm), you need to consider aiming for that by the end of your schedule / by the time you get to the interviews. One last thing, I would also recommend working on your resume, researching the firms you will be applying to, and drafting cover letters (if applicable).
Hopefuly this helps out! If you have any other questions feel free to let me know and don't hesitate to reach out for anything else I can help with.