How long did you spend for case prep ?

From start to finish, I spent exactly 3 weeks, and in that time I did 21 practice cases (with people), of which 3 with former/actual MBB people, placed strategically. In the end I got 1 offer from my top choice of MBB and I pulled out just before Round 2 for my 2nd choice of MBB.

I elected to do it in 7 steps:

1. Do 3-5 practice cases by myself over 2-3 days, just to get the very basics of casing.

2. Do approx. 5 cases with people, to get the feel for what that is really like, and get my first feedback.

3. Practice with a (former MBB) coach to see what I am currently worth, and see what big areas of improvements I have.

4. Work on key improvement areas with 8-10 more practice cases.

5. Practice more with (same) coach, to see whether/how much I had improved (did a full interview mock that time).

6. Do 3-5 more cases to really polish my skillset (and be more selective of my partners at this stage)

7. Finally, do a full mock with a current MBB M/EM/PL at my target to see what they really thought.

Then, I gave myself a few days off before doing R1 at the two MBBs where I had interviews, which happened to be on consecutive days. I had no more practice afterward, only the real deal, to avoid adding noise to the feedback I was getting. In the end (including actual interviews), I finished with 28 cases, of which 10 with actual/former MBB people.

EDIT: I should add that before I did that, I had watched several dozens (probably triple digits) videos on YT about casing, and read a LOT of articles. So, I did not go in blind before Day 1. But I only ever prepped once I got my interview invitations.