When to get Last Whisper

Almost.

Rush your main damage item. Grab your secondary dps item (typically phantom dancer on ADC's). Then grab last whisper. Get boots whenever convenient in between items.

You want to actually have damage before worrying about armor penetration, and by the time you grab last whisper is when melees start itemizing with armor anyway.

IE --> Pd/Shiv - > boots -> Lw

Pay attention to enemy builds. If none of them have built armor and none of them seem to have an armor build started, you can wait on the LW until 5th item. If the enemy is immediately stacking armor, get it 3rd item. If the enemy has a decent amount of armor, 4th item is the standard time to build it.