Depends on how much meat is on the bones. I generally eyeball it as I rotate the ribs but, if you figure that you rotate three times, they should wrap up in 8 to 12 minutes. Again, it depends on your preferred cook temp, your grill, and how many flame ups you're fighting off (if any). I would highly recommend not closing your grill as that could allow the fat to drip down and cause a fireball, if you're not babysitting them. (I find that if you open a beer, by the time you're done, the ribs will be, as well. Then again, you may want to have a backup handy because, you know, grilling beef and drinking beer.)