According to the U.S. theater-owner guideline, how many seconds long can a standard movie trailer be?

How to get close to this number

Reason from the audience. A trailer has to convey a premise and generate interest without exhausting attention, and a cinema shows several in a row before the film. Somewhere between two and three minutes is what the format supports, and the industry settled on a round figure inside that window.

The constraint comes from theatre owners rather than studios, which is the interesting part. Cinemas sell the time before a film and want to control how much of it goes to promotion, so the limit protects the exhibitor's schedule rather than the viewer's patience. Guidelines like this are usually negotiated between parties with opposing interests.

Exceptions exist and get approved for major releases, which tells you the rule is a convention rather than a regulation. Note also that the limit applies to theatrical trailers specifically — online trailers have no such ceiling, which is why the version you see in a cinema and the version on the internet are frequently different cuts of different lengths.