How long does a new moon last how many hours?

A "new moon" does not last at all. The phases of the moon as we see them are simply caused by the casting of the earth's shadow on the moon, thereby making only the lighted part visible, and this changes constantly as everything rotates. (Keep in mind we only see the moon after dark, which means the sun is on one side of the earth and the moon is on the other side - our side...therefore, our shadow is cast upon the moon.)

Romantically speaking, the new moon seems to last a few days after its actual occurrance, but this depends more on how long you want it to last than on actual visual evidence. If you are in love, any phase of the moon can last forever.