As much as you can fit into the box. No such thing as too much.

More than once!! ;)

:dewey:

Yes. Polly fill makes a box seem bigger to a speaker. You can make a small box act bigger therfore bringing a box closer to spec. You also can do this to lower the tuning frequency of a box.

My home stereo speaker are filled, even the enclosure for the mid. My sub box in the truck isn't but I've been meaning to since the box is too small for the sub.

I have seen older professional concert type speaker boxes stuffed. It works.