In my experience, practicing 15-20 cases is sufficient, if you do it strategically. My recommendation is as follows:
- Read up on the typical approaches and standard frameworks to get the concept.
- Then, do 5-6 cases to get a practical feeling for what a case is like. Start with easier ones - e.g. market size mini cases, simple profit tree cases, etc. This will help you develop a rudimentary sense for how cases work
- The next 5-6 cases should cover cases from all major types and help you gain the experience and comfort with standard frameworks and the thinking required for solving the cases.
- Lastly, you will want to do 6-7 cases to hone your skills. Practice with people who understand what they are doing - experienced interviewers, coaches, etc. that can give you 1-2 main items of feedback after each case that you can then practice to apply and improve on in the next case. During this time, you should also practice to move away from off-the-shelf frameworks and tailor, or - even better - develop your frameworks specifically during the case.
The further you move towards the final interview, the more important it is to practice with experienced interviewers. While you can easily ask any friend or practice with peers for the first few cases, you should aim for qualified, professional feedback as you approach the finishing line.
However, keep in mind, that this requires a strong plan and strategic approach to the preparation. I regularly see people doing 30-40 or even more cases. While this can also lead to success, in my eyes, it is a bit of a waste of time, especially for experienced hires that often also have a regular job to do while preparing for the consulting interviews.
Depending on your time delivery, you can then calculate the time you'll need. I don't recommend doing more than 1-2 cases a day, so if you can do 1 a day, you might be looking at ~4 weeks.
Let me know if this helps. I'm also happy to elaborate any of the above in more detail. DM me if you like.