I've done a bunch of interviews recently with candidates who sank their chances by not knowing when to stop talking.
Your answer to the interviewer's question should be direct and to-the-point. It should not result in you rambling on for five minutes, giving tons of background and tangents. If there's more to tell and you believe your interviewer would be fascinated, after giving your direct, concise (two minutes at the very most) response, you may ask, "Does that give you what you're looking for, or would you like me to go more in depth about this?" If the interviewer wants more, believe me, she'll say so.
You must also pay attention to cues. If your interviewer is looking bored, looking at her computer screen, or looking anything less than happily engaged, you might be rambling.
Rambling is the kiss of death because it turns the interviewer off, signaling that you're not good at picking up on conversational cues about where she wants to take the conversation, and raising doubts about your ability to organize your thoughts and convey needed information quickly.
But this is not license to turn into your opposite, the candidate who barely talks and makes me pull information out painfully, sentence by sentence. The middle ground is around one to two minutes per answer, unless you get the signal for something longer.