There is no rule that defines the precise length of a trailer. It can vary according to the desire of the director and the length of the production. The trailer for David Fincher’s film Millenium lasted at least eight minutes. However, it is advisable not to overdo it: the average attention time for a teaser is only a minute and a half.
Following too many excesses, in the United States the duration of trailers is currently capped at 150 seconds by the association that includes the main Hollywood studios. With a tolerance for only one per year that exceeds this duration.
The movie trailer Ambiancé is the longest in history with a duration of 72 minutes. His director has respected the proportions however: it is 600 times shorter than the film, which lasts 720 hours (a record).