How much Down Leak is normal?

It depends on the fabric as well as the seams. Down coming through seams is difficult to stop without sealing the seams. Down coming through the fabric can usually be pulled (not pushed) back in if you catch it early.

Are quills pushing through the fabric or are down clusters coming out the seams? Given the lightweight shell fabric, quills will come through. I'd recommend not pulling them out as it can produce a hole large enough to allow down to seep out. If the quill is really sharp, I sometimes use a nail clipper to snip off the tip and work it back into the bag… Anyway, you will lose quills, I usually see one every time I use an ultralight down bag. I see significantly less with my Wm Microfiber shell bags than with Pertex Quantum.

If down is coming out the seams at a consistent rate, you may have a bigger problem.

I figure that each time I use my down bag, I lose about 3 or 4 feathers.

Annoying, but nothing that I can do about it.

Some bags are listed as down proof, which I think means that they don't leak feather, but at the expense of either a heavier interior lining fabric or one that is not as smooth for comfort for skin to liner contact.

Trade offs.

-Tony

I lose a feather or two with each trip on my 2007 Pertex Quantum down bag.

I have an older down bag (1996ish) that is shelled with Gore Dry Loft and has never lost feathers through the shell fabric. The shell is considerably heavier, so much that at least one person confused it for cotton. I have lost individual feather fibers (not whole quills) through seams, through.

I have the same bag and get the occasional quills poking through and, much more rarely, a down cluster. There must be tens of thousands of feathers in the bag though so I wouldn't even give it a second thought. After 3 years, the bag still weighs exactly the same as when I bought it (on a scale weighing down to 1/10th of an ounce). If a feather isn't all the way out, just reach inside the bag behind the feather and pull it back through into the bag.

As Russell said – except that it might be closer to tens of millions rather than tens of thousands.
Yep, a few bits each night seems pretty normal.

Cheers

A few feathers each night is normal with that bag. If you see a half-poked-through feather, don't pull it out or push it back in. Rather, try to PULL it back inside by pinching the fabric behind the feather and gently working it back inside the bag. Pulling it out will only remove insulation from your bag and encourage holes in the fabric.