listen up, while adc's final builds are relatively static, the order is not.
get your defensive item when you need it, simply because putting out some damage is better than no damage.
thing is knowing when you actually need to get it, If I was going to just be general usually it is BT/IE->PD/shiv/TF(depending on adc and need)->LW->have at least a vamp scepter->probably LW->defensive. However if you have an actual appropriate need for it earlier, than pick it up. The biggest thing to learn about building an adc is what you need when, rather than the specific items that you want in your final build.
Do not get your defensive item for funsies, buy it only when it is appropriate to the situation. Your job is still to be that endgame glass cannon killer.
for example, if zed is blowing up your prostate in every fight, and you or your team can't appropriately protect you, grab a qss/warmogs/randuins/fM/etc.
If you are relatively safe with just your LS, peeling, and positioning keep getting your important damage items.
I know I keep saying it, but buy when you need it. This is extremely important.
Just because it has a million bloodthirsters doesnt mean it is bad. It works every time I use it. No armor penetration but you dont need it. One hit and you are full health again. Its amazingly cheap. No one can beat you. You can be burst down with a stun but warmogs lets you recover and one hit later you are full health. --- HAF 922 - i7 920 Nehalem @ 4.0 Ghz - 12GB DDR3 Patriot 1600mhz - ASUS P6T Deluxe - EVGA GTX 580 3GB - Cooler Master 850W