"Bless you" is usually said after a sneeze. :) Some people use it as an interjection: "And then my brother cooked dinner, bless him." In this case it's usually in a kind or thankful context. My British work colleague does this often, but in general it's not a common habit. The only time I remember hearing "God bless you" was when a class greeted the teacher: "Good morning Mrs Johnson, God bless you." ...is that still done nowadays?

When you can prove the existence of God.

"God bless you" you can hear after the church sermon, the preacher normally stands at the exit and blesses everyone who has attended the service. Hope, it helps :)

Sometimes people say "God bless you" after someone sneezes, but most just say "Bless you"

I always say GOD BLESS ME when I am in an enmergency situation.