The acceptable time you take when responding to a quite broad question of this sort will vary depending on the context. In my case, in one interview I took around 6-7 minutes or so to go through my entire response for a 'Why the firm' question and the partners absolutely loved it. That said, this was a rather exceptional situation. I had come prepared with a shorter version of my answer but I noticed I was getting very positive cues whenever I was deviating from my plan and going more in depth (including direct interventions from partners to support/add to my points) and as such I decided to continue doing that (while however making sure to check I was not taking up too much time and reading for any negative cues - such as partners checking their watches, seeing them get impatient etc). I think for a 'Why firm' question it is also the case that there is simply more detailed and impressive analysis you can always make, which is not the case for every explanation of a competency and experience (as here, more detail does not always tend to add something).
But to come back to your question, for a general 'Why you' question I think a response time of around 3 minutes should generally be fine. As such, I would advise you to prepare a version of your answer fitting that time frame. If you think this will necessarily mean foregoing to mention some relevant details, you can do what I did and prepare a longer-form version as well. Then, you can assess the context and simply make a judgement call on the day as to the level of detail you want to go into for every part of the answer.
