How far down is drain past p trap on bathtub already down 8inches past concrete slab no bottom still in sight?

How deep is main drain under tub

It can be any depth... depending on the situation and how the plumber installed the pipes.

how deep is main drain under tub
Did you mean, " How deep is the trap?"
Letmetellu is correct when he says it can be any depth. Let me explain why.
It all depends upon when the bathroom group is located in relation to the slope of the main. If the group is up at the high end you will see the trap. If it sets near the low end the trap will set lower and all you'll see is the raiser.
In some cases the plumber will 45 up from the main to bring the trap closer to grade. Regards, Tom