Only retain for 12-15 minutes. Too long and you risk absorbing it into your blood stream.
I've heard from folks who appear to be suffering from "Adrenal Fatigue" that they can tolerate coffee enemas with no ill effects, which is surprising to people who have learned that stimulants generally are dangerous for AF. I'm staying open minded about it though it seems paradoxical. I'm guessing there's some way it works well despite the stimulation. But while folks believe coffee enemas work well, no one has explained just how. With the belief that stimulants are bad we are left with the easy to conclude assumption that coffee enemas are okay because you don't actually absorb the stimulants. Sadly, we don't have good evidence to confirm this guess, or to point us at exactly what might be making coffee enemas tolerable.
The bowel is supremely vascular all throughout— there's no way coffee stimulants aren't getting into the blood.
So the next thing that people say is that the blood all goes directly to the liver via the hepatic portal vein system. So it does, and much of the caffeine is converted by this first pass processing to its first metabolite, paraxanthine, but that's just as potent as caffeine. Still, that said, if this hepatic portal vein and first pass metabolism stuff does in fact work, it is an improvement over regular coffee intake and having the caffeine floating around first for a while before then getting downgraded to paraxanthine. On top of that, with a coffee enema you don't have the coffee going through the entire digestive tract over a long period and absorbing all the caffeine — probably some of the caffeine in the enema gets flushed out if you're only holding the fluid in for several minutes.
So, I think coffee enemas might be several times more tolerable than regular coffee consumption, but I think to say that you don't absorb caffeine into your blood stream with coffee enemas is inaccurate. No offense.