Ask HN: How many customer/user interviews are enough?

Few days back I asked this same question, but it went into ether on this forum; hence asking again as this is, I think, fundamental to customer development. Sorry for the repost. The original question was:

When evaluating an idea how many customer interviews are enough to make a decision in favour or against the idea? Does it just depend on duration of the evaluation period or the number of total prospective customers interviewed, or both? How many customer interviews are enough and for how long should the idea evaluation phase last?

Thank you!