The Bravohood

I would say an an inch past the pocket of your pants. Make sure it fits and looks good with the pants you'll be wearing it with, not all pants are the same, and shoulders are key, the shoulder seem should not go past your shoulders

Well some make them shorter, and others make longer ones that completely cover your butt:





notice how some go past the length of the sleeves and some don't.

go for shorter longer looks bad

those three pictures you posted are very different styles of blazers, as you can see by the rest of the outfit of the person/mannequin wearing it. i don't think there's a set answer to this. it all depends on the style of the jacket and what it's designed to be worn with.

In a classic business suit sense (collared shirt, tie, slacks, etc) the old school rule of thumb is that the coat should go down to around your finger tips if you hold your arms straight down...not sure how this applies to modern casual sport coats, etc.