Since answers currently range from 1 to 2.5 minutes as "acceptable" (assuming you are talking about designing your own case framework at the beginning of your case interview), I thought it's worth commenting and adding some pieces of context/additional information.
First of all, there is no hard threshold time-wise (unless explicitly communicated by your interviewer). 3 things need to be aligned:
- Complexity of case outline and amount of information you have at this stage
- Amount of time you use for setting up your approach/structure
- Quality/depth of approach/structure you come up with
So - if the case outline is rather short and not too complex, and you come up with a somewhat ok structure but don’t get too detailled yet, even 60 seconds is probably on the upper end of time range.
However, if the case outline is very comprehensive with loads of information, and if you can present a highly focused structure on 3 levels, then also 2 minutes could still be within the range from the interviewer's perspective (even though really upper limit, since 2 minutes also represent already a significant percentage of total net case interview time).
Having said that, a few more comments:
- If you plan to use more than ~75–90 seconds of time, please make sure you keep the interviewer informed what you are doing every now and then - don’t let him sit in silence for 2 minutes getting bored watching you
- Sometimes interviewers tend to specify an exact amount of time (e.g. “sure, go ahead, you have 1 minute) - in this case you should do your best to stick to this time frame - at least keeping an eye on time and after time is over you can still try to get more time (and at the same time signalling that you did not successfully ignore the time constraint) saying something like “We are already at the 1 minute mark right now - could I just have a few more seconds to finish the structure?”. So now it’s up to the interviewer to decide, and you played it safe.
- The longer you take for defining your structure/approach, the more likely it gets that the interviewer becomes impatient and might even interrupt you saying “ok let’s see what you have by now and let’s complete it on the fly together”. The only antidote to that is keeping your interviewer entertained by informing him where you are roughly in your thinking and structure, to break this otherwise relatively long period of silence.
- In addition, make sure that you always start top-down, i.e. finishing a certain level of your structure before coming to the next one. Simple reason for that: if your interviewer suddenly interrupts you, you always have a (kind of) MECE structure to start with - and adding more details on the fly is easier than coming up with additional topics to be MECE on the same level.
Hope that helps!