Answer from Master Keisen
With practice, gently, it comes then that the practioners may spend more attention to exhaling, long breathing out observing it. The observation of the breathing is an important way to go deeply in your body, and of course also in your mind. But that requires some practice. It should come normally and in my perception one should not force it, especially at the beginning.
I would say go for a gentle breathing, naturally and unconsciously it will get longer at the same time as your body-mind quiets down.
One should not always take the rules of somebody else, you have to be intimous with yourself, feel your breathing, find your own way.
Mondo On the same theme : Zazen
To know more about mondo
You may want to see other themes of this mondo online, or ask your own question : see Mondo online : questions to zen Master Kosen Thibaut