After sunset it usually takes between 1 hour to to 1 hour and 40 minutes for it to get dark out after sunset.
The time ti takes for it to get dark after sunset will also depend on your location.
The further north you are, the longer it takes for true darkness to arrive after sundown.
But in my experience when I sit out and watch the sunset it takes between 1 hour to 1 hour and 10 minutes in my area for it to get dark after the sunsets.
The sunset times such as on sunset and sunrise websites or on Google are pretty accurate but they are only guesses.
It's so nice to sit out and watch the sunset at night and I love sitting outside at night as it's very peaceful and calm as well.