when to go all in post flop against one opponent

I always have a problem determining when is a good time to go all in postflop to protect your hand when against one opponent (assume that you know you have your opponent beat at the moment). Should the size of the pot in comparison with your stack size be considered and usually what pot size? Should you protect your hand immediately when there is a straight or flush draw? What other factors should be considered?

if you would bet or raise normally to protect, and said amount would be fourty percent of your stack... push it.

otherwise, if you have a read that says overbets scare them, or if you have 3-5 times their chips, an all-in is sometimes impressive.