Fuel usage all depends on so many variables - using water/sand/nothing, what's cooking, at what temp, for how long, weather - that there's no standard burn rate.
"Amatures" should just load up the ring full and not worry about it. Any unburnt fuel can be reused on the next cook.
With experience, you'll learn when to go with a half-ring or full-ring or anything in-between.
Remember - "It's better to have it and not need it than to need it and not have it."
You certainly do want to run out of fuel before it's done!