How long does slow-door photography usually take?

Slow door, also called B door, is a manual exposure setting. Usually used when the light environment where the real shot is located is very dark. It takes a long time to take pictures with clearly visible objects. Setting the time mainly depends on what kind of shooting effect you want. If you want to shoot the track of starlight, you need at least 120 seconds to see the track of starlight in the lens (of course, you need to shoot starlight in an environment without light pollution).

You can shoot more, and if the subject can't reach what you want, extend the exposure time.