How do the players know when to come in for a line-change? They can all be like spread out on the ice but at the same instant they all skate back to the bench. Someone once told me they though that players had something in their helmets that beeped when it was time to change lines, and the assistant coach controlled when it beeped. I don't think this is true, I'm betting it just goes by the flow of the game-- hopefully a hockey player can help me out with this (stupid) question.

thanks.

one of the ways to get a good line change is to get the puck in deep in the visitors offensive zone and change out your forwards, sometimes players will do their own line changes, you might see them wave their stick in the air telling the next guy in line to come out on the ice.
the coach can even yell to the players that its time to change up. usually a shift will only be about 20 to 30 seconds a game and most players play between 15 and 20 min. of a game.
there is nothing in their helmets that makes a beeping sound, so whoever told you that doesnt know anything. basically if you're tired and can come off then come off, usually you try to get youre next line out but sometimes that doesnt happen.
and no question is stupid, the only stupid question is the one not asked.

It might seem as though they are all just doing "whatever" out there, but they have plays that they memorize and strategies they employ. So they do know when to change based on how they've practiced. A player can switch off himself if there is an issue, but the line change where everyone comes off at the same time is probably part of the planned play (Like after the start of the game) or when the coach gives the word.

Sometimes the coach has a flashlight that he flashes in a player's face to tell him it's time to come off. No, I'm kidding, that doesn't happen. Haha...