(first post on the wiki)
Using a spreadsheet where the x-axis is gold spent on armor, the y-axis is gold spent on hp, and the result is your "effective health" (how much damage it takes before you die), I have determined these somewhat vague statements are true in most cases:
1) If the enemy is all AD, to be most gold-efficient, you should have 1 point of armor for each 7 HP. So if you have 2100 HP, you should have 300 armor before buying health is better. That's a lot of armor :P
2) If the enemy composition is the way it normally is (mostly ad, but still decent AP), then you should have about 1 point of armor or magic resist for each 14 HP. That means if you have 2100 HP, you should only have something like 100 armor and 50 magic resist before you should be buying health instead. This rule doesn't apply above 2600 HP; beyond that you should just be building more armor and magic resist.
In other words:
-HP over 3000 is useless
-HP as your only defense item is surprisingly inefficient: don't just buy warmog's.
-If the enemy is all AD, you should have 300 armor before you even consider buying health.