Hold times over 15 minutes are simply dangerous!
The body absorbs water in several ways. It is absorbing water a you retain the enema.
That's why you often have to pee in the late stages of expulsion.
If you try to hold an enema of more than a quart for more than 20 minutes, you can absorb so much water it overwhelms your kidneys, and water can build to such a level that it increases stress on the whole cardio-vascular system, and can cause it to fail suddenly.
I never attempt to hold an enema more then 15 minutes.
In a word...NO!
If the premise was that the water percentage and volume change associated with H2O getting into the blood stream was true, then anyone who has ever taken an IV of a liter or more of D5W would be dead. Furthermore, in a typical summertime run, I will be between 3.0 to 5.5 pounds lighter even after drinking 20 - 48 fluid ounces of water during the run. That is not body fat or glycogen that I burned. That's and equivalent of 1.35 - 2.5 liters of water. Go look at any 2-liter bottle and realize that what you are claiming is that half that volume is dangerous if not deadly.
The rate of absorption is not given by the volume but by osmotic diffusion that is governed by a lot of factors. At the simplest level, the concentration gradient of certain salts across the boundary between the colon and the bloodstream "controls" the flow. Hence, there has been an incredible amount of posting over the years about isotonic enemas. In theory, if your bloodstream were completely a static (not dynamic) system, if you balanced these salts perfectly, you'd be able to hold an enema indefinitely (ignoring the other peristaltic actions to move stuff along).
Point in fact, to answer the OP's question, I typically and routinely am able to hold a 3-liter water enema for more at least 30 minutes (usually more like 45-60 minutes). I often like to hold it longer for that simply because it feels good. The higher the volume, the more difficult it is to hold over a longer period of time. I have held 4-liter enemas for more than an hour (maybe once or twice per month) and on occasion have held 5-liter enemas for 20-30 minutes (those more than 4 liters take practiced focus). How much is absorbed over the course of the holding time and how much is retained for later absorption? Hard to say. But it comes down to between 1 - 2 liters of water simply by difference in mass before and after (noting that fecal matter also has mass).
@switchablesusie is likely to weigh in with a more informed opinion than my more basic one.
Edit: she already has.