Section 5: Rules of Play

Duration (Law 7, Sections 1 and 4). A match lasts two equal periods of 20 minutes of playing time; the clock is stopped by the timekeeper whenever the ball is out of play.

Source: Section 3, Section 1, Section 2, Law 7, Law 8, Law 11, Law 9, Law 15, Law 12

Send this to your friend — argument over.