On average, 29 1/2 days.
7 days
The cycle of the moon's phases runs 29.53 days.
The full lunar cycle is roughly 4 weeks, and from full-moon to new-moon is half the cycle, so it would take (roughly) 2 weeks to go from full to new moon.
Whatever moon phase you're looking at, it was the same 29.53 days ago, and it'll be the same again after another 29.53 days.
A full cycle is about 29.5 days, so a quarter cycle will be 1/4 of this, on average (divide 29.5 by 4). This is just an average; since the Moon's orbit is not a perfect circle, it may vary a bit.
7 days
7 days
The cycle of the moon's phases runs 29.53 days.
29.5 days
1 month.
29.53 days
29.5 days
28 days
29.53 days
29.5 days from new moon to new moon.
The full lunar cycle is roughly 4 weeks, and from full-moon to new-moon is half the cycle, so it would take (roughly) 2 weeks to go from full to new moon.
29.53 days after the last one.