Always learning - I don't follow any real formula on concentrating the stock. The nice thing is that it's flexible and you'll end up with a great product pretty much no matter what. Even the stock that I can is at least 2x concentrated in that it will turn to a gel in the fridge. I start in a 5 gallon pot, so have about 4 gallons of water to start, and cook it down quite a bit before taking the bones and scraps out.
So if I had two gallons of that, I'd take out all the bones, run it through a seive, and then let it simmer until it's reduced to about 2 quarts. So at that point, it would be about 8x concentrated, so a 4 oz portion would have as much flavor packed into it as a quart of the store-bought stuff. You're gong for something that is thick enough to coat a spoon, almost like a demi-glace. How long it takes to reduce depends on your method; you can keep the burner at medium and reduce it in an hour or so, or put it at a bare simmer, or let it go in the crock pot for several hours or overnight if you don't want to keep a close eye on it. I usually do the latter. When freezing stock, I don't bother to skim the fat out, because it's just extra flavor, so that's another step that can be skipped vs. canning.
