It means it costs them three months per year per employee. So, 'n' employees, n/4 years of man-power is spent fixing issues found by fuzzing. As others have sai

It means it costs them three months per year per employee. So, 'n' employees, n/4 years of man-power is spent fixing issues found by fuzzing. As others have said, FTE (full-time equivalent) is the more common name.