yes, definitely annual in the areas that get blasted by debris, like wheel wells and outside of frame rails, etc.
Inside the frame rails and body cavities, its not really quite as necessary to reapply
I have had it professionally applied to my FJ62 by the son of the creator of NHOU. Seemed to be a good product. I sold the vehicle a few years later, but worked as intended. Had a slight smell for the first week or so, like most rust inhibitors. Once it gets a nice coat of dust, it doesnt get on...