It's been a rather long time since I last tried and measured this. Most of my spinning is done for a specific purpose, and most of the time that is demonstrating or testing something - which means I usually spin smaller amounts of yarn, frequently of very thin yarn. So it's rather rare for me to try and pack as spindle with yarn until it's really, really full.
What is "full", anyways? Different people have different opinions about that. For me, it means that the spindle will not run easily and efficiently anymore when I flick it. The point where this occurs will, of course, depend on a number of factors: The type of spindle, the technique you are using to spin, the kind of yarn you are making, the density of the fibre, the density of the yarn, how you wind it onto the spindle, the placement and size and weight of the whorls you are using. I also find that with the growing cop, it can be necessary to adjust how I wind the yarn. At the beginning, it's helpful to spiral the leading thread up the spindle shaft before making the half-hitch. With a large cop, this can sometimes stop working so well; then I wind around the bottom of the cop, go straight up to the tip of the spindle, and place the half-hitch.
As my spindle fills up, I also replace the whorl I used at the start with a smaller, lighter one, so that the spindle turns quickly and nicely again. At one point, no whorl is the best whorl. At some point, the spindle, now whorl-less, is so full that it will be harder and harder to flick it into a speedy rotation, and that is when I declare it full.
In yesterday's test case, that happened at a total weight of a little more than 50 grams. The spindle stick comes in at about 8-12 g, depending on the stick, so that means I fit about 40 g worth of thread onto the spindle.
Which, if you ask me, is a pretty decent amount.