2009 IMC - 506.3.8 gives general guidelines regarding location of clean outs: Any portion of such system having sections not provided with access from the duct entry or discharge shall be provided with cleanout openings. "not provided with access" is hard to quantify.
With a flexible wand, a skinny arm, and a good back, perhaps a guy (or girl, if she's so inclined) can reach up through the hood connection, turn 90 deg. and reach down a horizontal about 10 feet. I sure wouldn't want to clean it that way, espcially since the water is going to want to come back at me.
Does anyone have more detailed, real world guidance on where to locate the cleanouts?