what ever you want it to be. its should be your personal preference. i think a good drop is no gap between the fender/wheel well and the tire..............where the fender ends the tire begins, but thats my opinion.
how low do you want your car, where is your location, what is the enviorment/climate you drive in? answer those questions before you lower it.
As stated above. It depends on application & personal preferance.
My fiance lakes my Accord because it's stock hight & she can drive it w/o worrying about scrubbing. I like my EJ1 because it sits low & corners well.