I use 1/2 to 1 lb of beans, roughly cracked, in my canister (in the keg) until I like the flavor then pull it. So for me it's not so much an exact amount as leaving 'em in there until I like flavor. Closer to a lb gets me to the threshold quicker.
EDIT - Usually the amount of time is tied to the intensity of the coffee beans. Columbian beans, several days, espresso beans, only a few.