An actor's screentime is the number of scenes they appear in, with the star of a film getting the most screentime of all.
If you're watching a show because your favorite actor is in it, you'll be disappointed to find she's a guest star with hardly any screentime. This word is also used for the length of a movie: "The screentime is over three hours; let's stream something else!" You can even use it to describe how many hours you spend looking at your phone, TV, laptop, or tablet: "I've had way too much screentime today. I need to read a book."