How Often Should A Kitchen Exhaust System Be Cleaned?

Below are various types of cooking establishments and their most commonly recommended cleaning frequencies.
 

Wood-burning or Charcoal-burning stoves, Chargrills, 24-hour restaurants, and some hamburger places30 DAYS
 

Many hamburger restaurants, fast-food locations, & Hotels: 90 DAYS
 

Average restaurants, employee cafeterias, hospital kitchens, pizza places, convalescent hospitals, small snack bars, oven hoods: 180 DAYS

Hoods over non-grease-creating appliances, such as steam kettles, dishwashers, soup vats, etc. : Anually