Is Zazen necessary? Of course not. Humans, dogs, birds, donkeys, grasses, the moon, my car – all get along completely fine without it.
What is zazen? This cannot be answered.
When is zazen necessary? Zazen is necessary when our own source calls to the surface, begging for truth, integration, wholeness and liberation.
How much zazen is enough? That is the wrong attitude. The question is, how much more do we have to give?
What about when we have nothing left to give? Zazen is the recognition that our life is a gift, and that we always have more to give.