It might seem like a trivial question, and most people would probably say 28 days. But, it isn’t so simple.
On average it takes 27.322 days (that’s a sidereal month, and a nice number of significant figures for something astronomical, especially when defining the day is not so clear cut) for the Moon to complete one orbit around Earth. However, the number of days between Full Moons is about 29.5306 days as the Moon has to “catch up with the sun” as it were. So, the actual number of days may differ from the average number by more than a half day. From one Full Moon to the next, the number of days in one lunation can vary between 29.272 and 29.833 days (another nice clutch of significant figures).
The age and apparent size of the Full Moon vary in a cycle of just under 14 synodic months, which is called the Full moon cycle.
The true Full Moon may differ from the calculated peak by up to about 14.5 hours, due to the normal irregularity in the Moon’s Keplerian orbit, and due to the periodic perturbations in that orbit caused by the Sun, the equatorial bulge of the Earth, and the proximity of other planets.
Anyway, I hope this little snippet answer the search query a recent visitor plugged into the sciencebase search box – “moon orbit earth how long”