You can find more about it at ISPD Guideline/Recomendations Peritoneal dialysis-related infection update 2010 PDI vol 30 2010

number of infections / patient years at risk = episodes of infection / year

7 episodes of peritonitis / 21.5 patient years
= 0.32 episodes / patient year
or 12 / 0.32 = 1 episode every 37.5 months (3.1 years)