How long would the exposure time have to be to create a star trail photograph?

The length of time required to photograph star trails varies depending on how long you would like the trails to appear. Generally, with a 10 minute exposure, you will have very faint trails. With a one hour exposure, you will have trails roughly four times as long. With a 180 minute exposure, you will have trails that are substantially longer, and will noticeably fill the picture. The longer the exposure, the more white the picture will become as it is filled with star trails.