Things have never declared themselves empty, nor have they declared
themselves form; and they have not declared themselves right, wrong,
defiled or pure. Nor is there a mind that binds and fetters people.
It is just because...
ZAZEN MEDITATION GUIDE - Chapter 11. How Long to Sit
If you are a beginner, a 15 minutes serious sitting is enough for each time you sit. If the circumstances allow, you can sit two or three times a day.
When you are ready, you can increase the time to sit up to the maximum of 45 minutes each time. If you do more than 45 minutes a period, the effectiveness of your zazen will be decreased.
Of course, when you are training at some Zen monastery or center, you must follow its regulations. You will be sitting whole day and night throughout the week of sesshin (Japanese, lit.: focusing your mind), is held there.
Sitting zazen is needed to be done on a daily basis, if you want get some benefit from it. Do not sit too much or too long in a period of time then quit it for another period of time (weeks, months... or longer). This way won't work as you want.