25-second Play Clock
At the end of specific official stoppages and game delays, the offense has 25 seconds to snap the ball to start the next play. If they fail to start the play before the 25-second clock runs out, they receive a 5-yard delay of game penalty.
40-second Play Clock
Teams have 40 seconds between from the end of the previous play to begin the next play. If a team fails to snap the ball before the 40-second play clock runs out, they receive a five-yard delay of game penalty.
Half Time
Between the second and third quarters, there is a 12-minute intermission where both teams leave the field and prepare for the second half of the game.
Injury Timeouts
If a player is injured, an official can call an injury timeout so the player may leave the field to get medical attention. Independent Certified Athletic Trainers called ATC spotters can also stop the game if it appears a player is injured and is trying to stay in the game.
Official Time
The stadium electric clock is the official keeper of game time. The game clock operator starts and stops the clock after a play is ruled dead. If the stadium clock malfunctions, the side judge will serve as the primary timekeeper.
Quarters
Regulation time for all NFL games is 60 minutes and each game is divided into four 15-minute quarters.
Timeout
A game may be legally stopped at any time, either by one of the teams or by an official. Each team is granted three timeouts in each half of a game with which that can strategically stop the clock. Officials can also call timeouts as needed to measure first down yardage, if a player is injured, or when playing time is being used up by an unintentional delay.
Two-Minute Warning
An automatic official timeout that occurs at the end of the last play that began before the game clock reaches the two-minute mark. The warning only occurs in the second and fourth quarters.
Source: NFL.com