It really depends on the amount of cooking that you are doing, and the type of fuel you are using to cook with.
Here in Los Angeles we follow the California Mechanical Code which states that “514.4.1 Removal of Contaminants Hoods, grease removal devices, fans, ducts, and other appurtenances shall be cleaned to remove combustible contaminants prior to surfaces becoming heavily contaminated with grease or oily sludge. [NFPA 96:11.6.2]”
This kitchen exhaust system is an example of one that is contaminated and overdue for a cleaning
There is also Table 514.3 which lays out a schedule for inspection of grease buildup:
|TYPE OR VOLUME OF COOKING||INSPECTION
|Systems serving solid-fuel cooking operations.||Monthly|
|Systems serving high-volume cooking operations
such as 24-hour cooking, charbroiling, or
|Systems serving moderate-volume cooking
|Systems serving low-volume cooking operations,
such as churches, day camps, seasonal
businesses, or senior centers.
Ultimately it is the restaurant owners responsibility to have their kitchen exhaust system cleaned on a schedule that keeps the system free of heavy grease buildup.