It is replaced after every period and after every tv commercial break. Another time they would replace it would be if it goes out of the rink. If a goal is scored and has significance to a player, that player may keep that puck, which would result in a new puck being used. They have to ice pucks down before a game, if the puck becomes to warm, it will bounce and roll all over the place.
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There are a couple of circumstances that would call for the replacement of the puck during a hockey game. The first and most obvious reason would be when the puck leaves the playing area, and goes into the stands or is unrecoverable. Then next would be if during the course of the game the puck is damaged to the point where a large chunk or gouge is in the puck, which would effect the way the puck moves. At this point the Referee will determine that the puck is unuseable and switch it out. The other instance where a puck may be replaced is more based on ceremony and skill. Normally when a player scores his first goal for a team, he is allowed to keep the puck as a trophy. This also sometimes occurs with hat-trick goals, and milestone goals, such as 100th goal, etc.