Soccer referees have full discretion to decide how much stoppage time to add to a half for any time lost during the period. Because of this, the amount of time added for a particular event can vary widely from game to game. For example, if play stops to assess an injured player's condition and remove him from the field, one referee may add four minutes of stoppage time to the end of the half, while another would add only two minutes.
This level of uncertainty doesn't exist in American high schools and colleges, which use a countdown clock just like other sports. The length of a soccer game also depends on the age of the players. U.S. Youth Soccer provides guidelines with shorter periods for players under age 16. For example, games for children younger than 6 are divided into six-minute quarters.