A quality that is required in medical professionals that are under pressure is an ability to be concise without sacrificing the quality of the communication. As such, your answers should fall into the category of being long enough that all the relevant experience and evidence for your answer is communicated, but not too long in that the interviewers lose interest and you begin to repeat yourself. The answer is not precise and depends on the context, but for many questions this will be around 1-5 minutes. The exact timing can be very difficult to get right - as we know, but with practice this will become much easier. If in doubt, er towards a more concise answer.