no problem bro...it is just my own experience that i would like to share with.
how long? depends. i notice a few B8 been using spark plugs from the halfcut. i dont do that. anyway, it is up to individuals.
myself, when i was using the NGK BCPR6ES-II, i'll change it every 9000km. it's not a recomendation, just to give u idea when u wanna change the plugs. better still, if u'r thinking of changing the plugs at every services of engine oil, that's much better for the engine.