I've heard it said that 16 weeks is the time frame to really see the results of a food change. I usually see changes more rapidly than that, but its probably a good general range. If you've ruled out external parasites and environmental influences, maybe you should pick a more high-quality food? There are grain-free diets out there that, while expensive and not normally suggested for puppies, might help with the itching.
Lindsay & the Gravity Dogs
"Though she be but little, she is fierce." -- Shakespeare