During the game, goalkeepers will often remove their helmet. This is because it gives them a better view of the action and also stops the helmet infringing on their shouting (which helmets often do!). If you ever watch Ali McGregor (GB and Loughborough Students no 1), he will very often remove his helmet when the ball is outside of his zone, so that he can heard clearly by his teammates when shouting out advice to them.
The reason goalkeepers like to remove their helmet for two reasons: a better view of the pitch and not being blocked when they shout. Extra vision means they get a better look at what is going on in the game, and they can therefore give better instructions to their defenders as to what to do (open players to mark) and where to go. Masks often block the mouth and therefore limit the sound the goalkeeper can make; muffling their voice when they shout. By removing the helmet, the goalkeeper can shout instructions at their defenders more clearly, so they are properly heard.
When you take off your helmet depends on where the ball is on the pitch: if it is just outside the D, then you’re obviously not going to take it off as a scoring chance could happen! When the ball is outside the half, it is safe to take off your mask. The further the ball is in the opposition’s zone (like their 16 or within their D), the longer the time you have to stand around without your mask on.
Putting it back on!
As soon as the ball gets outside of the opposition’s zone and towards yours, it is necessary to put your helmet back on. Make sure you do it instantly; putting it back on as soon as the ball comes loose and crosses your half into your zone, so that you have time to get ready and set in your stance before the opposition have the chance to score on you. You don’t want to leave yourself vulnerable to taking a shot off the head!