How fast does it get dark after sunset?

How fast it gets dark after sunset varies with the observer's latitude and the time of the year.

The diagrams show the Sun's altitude before sunrise or after sunset. Negative angles in the vertical axis mean that the Sun is below the horizon. The different colours indicate the Sun's altitude at a given number of minutes before sunrise or after sunset, as shown in the legend.

Explanation of numbers shown as degrees: An altitude of +90° means that the Sun is in the zenith (directly overhead) and 0° means that the centre of the Sun is in the horizon. Similarly, negative angles mean that the Sun is a certain number of degrees below the horizon.

It is very hard or impossible to explain in an exact and understandable way how dark it is at various times before sunrise or after sunset. Instead, a preferred solution is to observe the sky and landscape personally and compare it with the Sun's position below the horizon as taken from a table or diagram. Keep in mind that clouds affect the brightness of the sky.

Some general rules are as follows:

When the Sun is less than 6° below the horizon, it is possible to read a newspaper outdoors under a clear sky. Jupiter and Venus may be possible to observe if they are above the horizon and not too close to the Sun. The brightest stars appear when the Sun is about 3° below the horizon, but their visibility actually depends on the observer's eyesight.
When the Sun is 12° below the horizon, a lot of stars are clearly visible. The sky is perceived to be fully dark, but a blue shade is easily seen near the horizon above where the Sun is.
When the Sun is 18° below the horizon, it is completely dark, and the Milky Way and deep-sky objects may be observed.