In my experience it is less a question of time, more about the number and quality of cases that you are doing in preparation. Depending on your available time (e.g next to a 60h job) this can take anywhere between one and 3 months I would say. Here's my best practice for how to plan cases:
In my experience, practicing 12-15 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 3-4 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 rudementary 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 4-5 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.
Keep in mind that if you prepare with peers here on preplounge only the cases that you do as candidate count, not the ones that you do as interviewer. But: Doing cases as interviewer of course also has a learning effect as you see what behavior works and what doesn't, so I definitely also recommend doing a few of those.