Is 15 minute session per day of Zen Meditation too short?

Yes. You need at minimum an hour a day if not longer. It all depends on how serious you are about enlightenment.

When you reach “no mind.”

Jesus — died for us because He loves us ! How about praying to Him instead of meditation on Zen ?

Ten pounds of flax.

I guess it is up to the individual to decide…

Once you can alter your state at will,any amount of time is fine. You could do 5 mins. several times per day. The key is meditating long enough that you go deep as soon as you begin.This can take a long time depending on your ability to empty your mind.

All depends on how indoctrinated you wish to become in something that is nothing but hype.

The usual recommend is 20 minutes for a general benefit. However to get the FULL benefit you need much more.

quality, not quantity

doesnt that depend how loud the clapping is

We need to still our mind only then we can control them.
15 minutes is fine if you are a beginner but when you grow in it you care less about time.

If one is just starting to sit zazen, 15 minutes is a good starting point. The main goal is to practice everyday. Then each following week, five minutes can be added to ones sitting. This is done until sitting comfortable for 30 – 45 minutes is achieved.
A zendo sitting schedule could look as follows:
25 min. Zazen (sitting meditation)
10 min. Kinhin (walking meditation)
25 min. Zazen (sitting meditation)
The entire purpose of practicing zazen is to develop strong concentration and mindfulness in the present moment. These traits are then applied off the cushion into everyday life. One practices zen to include everything within and around oneself, to be mindful of the world as it is in the here and now.