system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:why most movies' runtime is around 110-120 min? Is that just a custom we're used to or is there any pragmatic reason behind that?

Like, because our concentration only can last for around that time? Or because it's thought fitting ticket's price?
gpt
A standard film length does not exist. There is no maximum length for a feature film, but there are multiple definitions regarding minimum length. The American Film Institute Catalog of Motion Pictures and the British Film Institute both define a "feature film" as having a run time in excess of 40 minutes. The French Centre National de la Cinématographie uses the definition of 35 mm film longer than 1,600 meters (58 minutes and 29 seconds).

Since the late 20th Century, many film editors in the United States and United Kingdom have referred to Alfred Hitchcock's advice on movie length:

"The length of a film should be directly related to the endurance of the human bladder."

Hence a common run time of 80 - 120 minutes.