More on breath and zazen

From here:

１回の呼吸が１０～１５秒程度です。これで用意万端です。
次に、雑念に振り回されないため、呼吸の数を数えると良いでしょう。
線香１本の燃える時間２０～２５分を一炷（いっしゅ）といい、坐禅の時間単位です。

1 complete breath [inhalation and exhalation] takes 10-15 seconds. This [along with correct posture] is sufficient preparation. Next, in order to have idle thoughts not fly about, counting breaths is a good idea. For about the time it takes one incense stick to burn (20-25 minutes) one should sit in zazen.

Now the above breath rhythm is good for other breath practices as well, and when combined with the breath instructions below is a good overview of what you want your body to do when in zazen.

It takes years to get it right for a sustained period, but once done, is very useful.