I've posted here before, but thinking back about 50 years I'll pass along some observations. First my comfort level is about is 2500cc (2.6 quarts). I can do 3 quarts, and got to the gallon club once or twice, both times with very low flow through a deeply inserted colon tube, I didn't think it was much fun
The first woman I regularly did enemas with took two quarts from the outset, and that was her comfort level, any more and she would start to feel nauseous. Even after we split we would occasionally meet and have enema play, and that was the amount she was comfortable with. She was of slim build, 5' 6"
The next woman, even though she had been given a barium enema and had taken, by her observation, most of it when she was about 18, when we started out she had trouble taking much, and was most comfortable with a couple of douche syringe bulb fulls. Didn't like bags at all She was 4' 11" and average build. Now fast forward 5 years, she doing 2500 cc as easily as I and liked the bag high and fast flow, and lots of soap. She would routinely request a super-soaper. When we got an E-Z-Em Super bag with the large diameter tubing she was into knee-chest and as fast as it would go in. And, funny how kink develops, from being a bit uncomfortable if I was in the bathroom with her when she expelled to sitting on the edge of the tub and expelling so she could see everything that came out and wanting me to watch.
Most all of my partners, and there have been a few over the years, were happy with 2 to 2 1/2 quarts, but one achieved almost 6 quarts at an eFest in Las Vegas in 2001 or 2.
So, in my observation over the years, 2 to 2/2 quarts is comfortable for most people, but, as they say, your results may vary.