I find LW to be an incredibly powerful item for an adc, and should certainly have some priority over other items, but I find it hard to fully judge when you should build it.
With my experience as an ADC (I do not main this role, however I do enjoy it) it is rather situational as to when you should plan to build LW. I have never ever seen it valuable to build this item in the early game as an ADC, but after you examine enemy build paths, mid game is where you should start examining when to purchase a LW.
With my experience at hand I try to value sustain a little bit more in the early game, than shredding tanks with the percentage of armor pen. I will try and give you an idea in terms of where I decide to build LW because at some point or another, it will be in my build.
Depending on the type of ADC I am playing, my first item actually varies.
Normal ADC meta insists buying a BF Sword item first (i.e; Blood Thirster, Infinity Edge)
But you do have circumstances where say a Vayne's Priority build path insists you buy BOTRK first, or say. Corki, who thrives off a Trinity Force for first buy. Those are all pretty standard and most ADC's obviously know this.
It's mid game and your 2nd, 3rd, and 4th items that really take some thinking. (excluding boots which are always prioritized to finish right after your first item)
A player like myself, who examines team and enemy build paths, I like to use my situational thinking to decide what item to build next. My 2nd item varies, depending on the champion I am playing, and whether or not I am ahead, so-so, or even behind.
If I am ahead of the enemy ADC and I strategize well, I will pick up a Vamp Sceptre for the extra damage, and the life steal, and will prioritize a Zeal item for the attack speed, finishing the life steal item afterwards. If I am so-so or behind, I will always finish the life steal item first. Playing a champion with AS buffs like Graves and Tristana, I will actually build the lifesteal item first regardless.
Now at this point in time you have a minimum of 2 or even 3 items and your tier 2 boots. This is where you want to examine enemy team builds for armor and snowballing champions who are trying to survive by building armor from a fed ADC.
In my experience of playing the ADC role, I have seen it incredibly efficient to purchase a LW anywhere from your 3rd or 4th item and finish your build off with whatever else you need and a. GA.