How much time exactly is not just depended on you, but it also depends on the interviewer. Because a lot of interviewers like to ask follow-up questions on your stories, and sometimes for the single stories the interviewer can spend the entire FIT part (it's around 20 minutes).
However, what I can surely tell you that, when you are answering, every answer you should try to restrict within the 3 minutes. Because psychologically, more the 3 minutes of a monologue is not good communication, because the opposite person loses his concentration.
So, how does that pen out?
For example, if the interviewer asks, "Can you tell me a story where you demonstrated entrepreneurship spirit?", so make your answer usingSTAR model. That answer shouldn't be longer than 3 minutes. It should be exhaustive to cover the story, but it shouldn't be too long. However, be prepared to answer the follow-up question, if an interviewer specifically says: "Ok, so why don't you tell me, what exactly you did in this particular part of the story?".
Again, your time will start, and make sure that this answer is in line with the structure and don't talk longer than 2-3 minutes.
This series of conversations sometimes gets to the entire FIT part, or sometimes interviewers ask different questions.
Does it make sense to you?