I've gotten to 3 quarts a few times, but just under 3 (a full 2500cc E-Z-EM bag) is usually more than enough. Just over 2 is best for my pleasure.
Now that JBL comment.... this is an observation, I think your colon will take more when it can't stop the flow due to the pressure.
I have one of those flow indicators on one of my bags, and even with the bag hung high, say 5 feet, I can stop and reverse the flow. With a JBL or shower shot the pressure is high enough that isn't going to happen, and you take more. Obviously the down side.... you could do some damage.
You might also be opening the ileo-cecal valve and getting water into small intestine.
When some experiments were done with retrograde enemas, they could always tell when the small intestine was full because when solution would flow into the stomach the patient would vomit.
I have a couple of the Italian made clear buckets (I love them because I know EXACTLY how much solution is in). At an E-Fest a few years ago my enemate came in second taking 5+ quarts, she is about 5' 4" so that amazed me, of course I had really cleaned her our first. The winner, a male, about 6' and probably close to 500lbs took over 6.
A few weeks later she wanted to try to see if she could get to 6 quarts. This gave a whole new meaning to "blowing it out of both ends." She didn't quite get to 6, and after the ordeal the only side effect, she was starved.
I have a couple of 1000cc piston syringes, always easy to get a fill with no cramping, but it is also interesting to feel the back pressure on the syringe handle.