Lunation is the mean (average) time for one lunar phase cycle (i.e., the synodic period of the Moon, or the amount of time from one new moon to the next).
In the year 2000, the average length of a lunation was 29.530588 days (or 29 days, 12 hours, 44 minutes, and 3 seconds). However, the length of any one synodic month can vary from 29.26 to 29.80 days due to the perturbing effects of the Sun's gravity on the Moon's eccentric orbit.