When cooking bone broth how much should you reduce?

However much you want.

Usually at least by half.

Depends on what you want.  I don't do much bone broth but I do a lot of stock for cooking purposes.  It's hard to tell you exactly how much to reduce it because it depends on so many factors, like how much animal matter and water you started with, your simmer temp, the length of time and your intended purpose for the stock or broth.  I do a less reduced stock for cooking - it will still gel up, and an intensely reduced stock for sauces, which I keep in 1 - 2 oz cubes tightly wrapped in plastic then in a freezer bag.  I make that intensely reduced stock 2-3 times a year for sauce purposes, I make regular stock or broth 2-3 times a month.

You want to simmer it for several hours, but realistically, until you like the taste best. At some point the flavor is not going to change much.