The answer to this question really depends on the type and length of the project you are writing. Generally, a prologue should convey enough information to give the reader a sense of what they can expect to read without being overly lengthy. If you are writing a novel, a prologue should not be longer than two to three pages. If it's a short story, the prologue should not exceed one page. For a screenplay, a prologue should roughly be no more than a couple of minutes long if it is to be included in the film.
Ultimately, though, the key thing to bear in mind is that the prologue should hold the reader’s interest and quickly move into the story. The primary purpose of a prologue is to grab and keep the audience interested in what is to come, so all the elements that you include in a prologue should be related and pertinent to the story. Keeping it brief and focused on the immediate story helps to ensure that your reader will continue with the rest of your work.
