Both of these hoods were cleaned at the same interval. Talk with the guys who have seen many varieties of dirty to determine what service interval best fits your needs at a fair price.
NFPA 96 code 184.108.40.206.1 states- Upblast fans shall be supplied with an access opening of a minimum 4 in. on the curvature of the outer fan housing to allow for cleaning and inspection of the fan blades.
Outside of being code, this access is amazing for a hood cleaner. This fan was wobbling due to the outside build up, after the access was installed the blades could be cleaned properly. This access install cost much less than a new fan or any downtime.