how the duration of unemployment is derived?

[XIPP]

The average monthly flow out of unemployment pool of 7.0 million people each month is 3.1 million. Put another way, the proportion of unemployed leaving unemployment equals 3.1/7.0 or about 44% each month. Put yet another way, the average duration of unemployment - the average length of time people spend unemployed - is between 2 and 3 months.

Now, I can't understand how the duration of unemployment is derived and claimed to be 2−3 months.