A pothead can get high for about 4-5 days with a quarter bag, 28 grams. A quarter bag will roll about 12-18 joints. And they are not big joints.
Or you can say, a pothead typically smokes half a joint to get high. A high lasts 1 1/2 to 2 hours.
Your math is a bit off. Your estimate is at least twice too high.
You can get high off of one hit from the stuff that is around today.
Smoking marijuana lowers the eye pressure. But the effect of smoking marijuana lasts only about two hours or so. Therefore, the marijuana patient must smoke every two hours or so to maintain a low intraocular pressure. Doing the math, this would require 12 joints. Yes, if it takes half a joint to accomplish the effect, then the glaucoma patient would require 12 half-joints or 6 full joints per day, every day.
But, this depends on the amount of THC in the marijuana and the amount of marijuana per joint. And I am not aware of one single study which calculates how much of either is necessary.
With a patient's eyesight at stake, I don't understand why people even consider this type of treatment.
"...a quarter bag, 28 grams."
You do mean seven grams, don't you? And do you think the average pothead smokes seven grams by himself every four or five days? Some smoke more than that in a day, but my guess is that most smoke a good bit less than that in four or five days. From our national drug use surveys our government estimates that the average person who will admit smoking pot in the past thirty days smokes about seven grams per month. Holland did a study on how much current pot smokers were smoking and they came up with an average of about ten grams per month.