This question is really puzzling for me.
My cat is comfortable with any depth and has never shown any problematic behavior.
She is not even concerned with the company that makes the sand, and any sand is OK with her.
However, if the sand depth is too much, it gets difficult for me to clean the litter box. This is because my cat has the habit of digging a lot.
On the other hand, if the depth of the sand too little, the solidified clump of urine often gets stuck at bottom over plastic surface of the litter box, because my cat digs so much.
Also, if the depth is more, my cat has this tendency to sometimes just go and sit over the sand to relax.
Note: it is not that there is lack a of comfortable material at all. There are different heights and places for my cat to relax. It is just that she sometimes thinks that the litter box sand is the ideal bed for her.
So I am wondering how much depth is ideal for a cat?