Whatever your level of skill, breathing is a technical factor that you need to work at. Breathing properly guarantees a good position in the water and tires you out much less. Learning to breathe is made up of two main phases: breathing in and breathing out. You need to quickly breathe in so the least possible disruption is caused to the horizontal position and the balance of the swimming stroke. Depending on the swimming stroke, it is done on the side (crawl), or ahead (butterfly, backstroke, breaststroke). To make your inhale as effective as possible, you must firstly fully exhale all the air in your lungs. The exhale is therefore longer and deeper than the inhale. Lastly the breathing must be coordinated with the arm and leg movement to ensure swim stroke stability.
Breathing in starts as your arms have finished their inward sweep phase. The head is raised looking straight ahead of you. It must be done through the mouth and be short so that the head can regain its position as fast as possible. It is done with each breaststroke movement.
The exhale is done simultaneously through the mouth and nose as soon as the head enters the water. It must be long and continuous.
Just like breaststroke, the exhale starts when the head goes back in the water. It is long and continuous and is done through the nose and mouth at the same time.
Just like breaststroke, the inhale is done in front and must be short and effective. The chin mustn't be out of the water so the least possible disruption is caused to the horizontal position and the balance of the swimming stroke. The taking in of breath occurs at the start of the arms' aerial return phase. The head must be repositioned as an extension of the body before the arms have fully returned to the front.
As always with the goal of maintaining the most aerodynamic position, breathing is done every other stroke.
Taking in of breaths in backstroke is easier to control because is requires no particular head movement and doesn't destabilise the balance of the swim. You must however follow a few small rules to properly oxygenate the muscles during the swim.
The exhale phase must be as long as the inhale phase. To prevent hyper ventilation, it is recommended that each lasts two arms strokes. The exhale must always be complete so the inhale is more effective and provides the body with the maximum amount of oxygen.
Breathing in crawl is much more difficult to control because it necessitates turning the head alternatively to the right and left and therefore disrupts the swimming balance.
The taking in of breath must be short and positioned at the end of the arm push. The head must then be repositioned as an extension of the body as fast as possible. It is not necessary to lift the whole of the face out of the water, but simply to bring the mouth to the surface to be able to breathe in.
The exhale is yet again long, continuous and full, and starts as soon as the head has regained its position in the water.
The breathing cycles vary according the swimmer and the length of the race distance being done. Generally the taking in of breath is done every three arm strokes to the right and to the left. Some prefer increasing the cycle in order to maintain the best position to favour balance and for keeping abs and glutes tense.
Yannick's answer:
"It is well-know: the less you breathe, the less you slow down. It is however important to manage your oxygen intake so you don't run out of steam in the last few metres! Everybody has their own technique, but the most commonly used technique consists of taking in breath every four arms strokes for the first 50 m, then every two strokes during the second half of the race."