Hello, slightly new to league. I haven't done any extensive testing, mostly just math on paper. and mostly reading about LW on the [Last Whisper WIKI](http://leagueoflegends.wikia.com/wiki/Last_Whisper). It looks like it isn't gold efficient until AT LEAST mid game. against an ADC, which would generally have around ~50 armor mid game, you'd need at least 180 attack to break even on gold efficiency. From what i see, it wouldnt be any good until a 3rd or 4th item ? maybe after Infinity Edge, Blood Thurster, Phantom Dancer, then Last whisper ?
A friend told me he usually get LW as his 5th item because at that point it gives him a big spike in power and then he goes for a Guardian Angel or some other defensive item.