When I used cold brew for my coffee porter, I started with about a quart of water (32oz), and 8oz of ground coffee beans. After straining and filtering, I was left with a bit less than a pint of cold brewed coffee that I added to the keg.
Since then, I've changed my method and simply add whole beans directly into the fermentor for 24 hours before I keg. The results are just as good, and I save time.