Have you checked your Insurance policy?
To comply with Australian Standard AS 1851-2012, Insurance companies require that a full exhaust system clean be performed at least every 6 months. Typically as Lumley Insurance points out, one of the catering industry’s leading underwriters, “The ducting behind the filters and all the way through to where the duct discharges into the open air, including the extraction fan and motor, must be professionally cleaned at least every 6 months”. (Ref: Lumley Insurance Property and Liability/Restaurant and Café)