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.