I have found that the best time to add ginger is in the last 10 minutes of the boil, too early and you boil off all the flavour. I have tried using thin slices and grated ginger and find that grating works best for me. As for how much to add, that depends on your recipe as well as how much ginger taste you want, but I find that 4 oz grated at 10 minutes to flameout gives me a pronounced ginger taste in 5 gal. of a pale ale.