How often is a blue moon? A blue moon is two full moons in the same month.  How many times will a blue moon occur in the average lifetime of say 70 years?
As months are not of fixed length it is only possible to calculate an average.

From Wikipedia:

The average frequency of a blue moon can be calculated as follows. It is the period of time it would take for an extra synodic orbit of the moon to occur in a year. Given that a year is approximately 365.2425 days and a synodic orbit is 29.5309 days, then there are about 12.368 synodic months in a year. For this to add up to another full month would take 1/0.368 years. Thus it would take about 2.716 years, or 2 years, 8 months and 18 days for another blue moon to occur.

So for a 70 year life span the average number of blue Moons would be 70/2.716 or 25.77.