The official length of a game is 60 minutes. This time is divided into four 15-minute quarters.
There is a break of two minutes between the first and second quarters as well as the third and fourth quarters. A longer, halftime break occurs between the second and third quarters. Halftime usually lasts 12 minutes, but it can be extended in the playoffs.
Stoppages of Play
Even though one quarter's time limit is 15 minutes, the clock doesn't count down continuously. The clock stops any time an incomplete pass is thrown, a player runs out of bounds, during changes of possession of the ball, and after a team scores.
Teams can call a timeout to stop play three times in each half. Officials can stop play for circumstances like player injuries and the need to review certain plays on replay. There is also the two-minute warning: an official timeout with two minutes remaining in each half.
TV networks will use a stoppage of play to air commercials. Two common times for this are following a score or a kick to the other team. This often doubles the time such a break would usually last, which would be about 30 seconds.
If the game is tied after regulation, the game continues in "sudden-death" overtime (the game will end the next time a team scores). In the regular season, a game ends in a tie after a maximum of 15 overtime minutes. In the playoffs, the game continues until someone scores, thus winning the game.