How many times do you suppose to knock on a door?

twice

Sheldon says 3 time, pause then 3 time, pause then again 3 times.

Until squidward screams

Do you watch "Big Bang Theory"? ... Sheldon always knocks 3 times, and Sheldon is th expert on all such things.

Typically you should knock 3 times. Its is a psychological fact that knocking 3 times alerts the person better then knocking 1 or 2 times. Knocking anymore than 3 times is considered warning or annoying. :)

3?

three times?

I would say 3 knocks with a 4.5 second break between each knock.

In mathematics category? How many times ARE you SUPPOSED to knock on a door? A few times should be enough to know if they will answer, or if they are not at home. Do you suppose that knocking all afternoon will be productive? See the difference?