I've been into enemas for over 25 years :) When I first started I couldn't take much more than one full 'standard' 2qt bag. But after several months, I was able to take 3qts (plus a little) before calling it quits. Now, after a quick 3qt clean-out, if I let the enema rush in for the first 2qts and then slow it down I find that I can easily take 4qts with no problem. I like to enjoy that fullness level - which for me is just a notch above "gee, did I actually take it all?' but not quite a 'omg, I think I'm going to burst!' If I'm exceptionally horny, :P I might attempt to add more water to the bag and take just a bit more...the max I have ever done would probably top out at 4.5 qts.
On a slightly different topic: I've scheduled an official colonic session this weekend, so I'll report back here (in a different thread) how that goes.