In a given month, what percent of nighttime hours are available for observing without interference from the moon?
It depends on how much interference from the moon you're willing to put up with. If the moon is just a crescent, you might be able to observe something that you couldn't if the moon was full. But to simplify the question to "What percent of the nighttime in a month is the moon up at all?", the answer is that it's exactly half.
This page was last updated on July 18, 2015.