The largest ones I've held for 30 minutes or more have been:
1) 4.3 liter baking soda enema held for more than 60 minutes (estimated time was 85-90 minutes). It was the last enema of a series. Fell asleep on the bed with the retaining nozzle locked inside of me while watching Sports Center. After emptying the bag hung on the bedpost and closing the clamp, I watched Sports Center for a while. The enema was warm and soothing and baking soda enemas tend to allow the colon to relax as opposed to trying to expel the liquid. Knew it was more than an hour because I woke up to a segment that I had "just watched" the previous hour and then a segment that I hadn't seen (because I fell asleep). I was awakened by a sense of pressure in my rectum pushing the retaining ball against the inner sphincter muscles. But the locking ring on the 2.5 inch retaining ball held the nozzle and retaining ball firmly in place.
I relaxed for a few more minutes and allowed the urgency to expel grow before I finally swung my legs over the side of the bed, lifted the empty bag from the bedpost and walked into the bathroom with the hose hanging from my ass. I hung the bag from the towel rod, added 600 ml of very warm water to the bag, sat down on the toilet and opened the valve slightly to allow the water to flow in slowly. Because I was using a long colon tube, I could feel the cold water in the hose first followed by the very warm water near the hepatic flexure. The bag drained again while I masturbated to a very strong orgasm. After the orgasmic release, the urge to expel became much more pressing and I "pulled the plug" and the long tube out of me. After I drained the enema, I climbed back in bed under the warm covers and slept deeply.
2) 3.2 liter SSE w/ 10% glycerin held for just under 75 minutes with a silicone double balloon nozzle. This was not (at all) the easiest enema to hold for this period of time. The total liquid volume I placed in the bag was 3.5 liters (350 ml glycerin, 3150 ml warm water, 45 grams of grated/shaved Ivory soap). I didn't drain the bag and only took what I thought I could hold for a while (measured the what was left in the bag to determine the volume I filled myself).
I know this enema solution well and know what to expect most times. Usually at the 8-10 minute mark after I've filled myself, I get the first "kick" where the peristaltic contractions (cramps) are relatively powerful. After the first 3-5 of these contractions, the power of the contractions fall off to a more manageable and rhythmic level (at about 15 minutes). It's at this time (I imagine) that the enema solution has sufficiently filled my colon and is in the process of softening and/or dissolving the lumps of fecal matter in my colon because it seems like there is more free-flow of the enema solution in my colon during each wave.
It is at this time I can just relax and go along for the ride. The double-balloon silicone nozzle fits much deeper into my rectum than the latex ones I own. Consequently, there is more pressure and stimulus on my prostate both as a matter of the balloon inflation and the positioning of the nozzle tip. The soap/glycerin solution provides enough lubrication that there may be just a bit of sliding motion as each wave of contraction passes down the rectum past my prostate. This stimulation usually causes an intense erection and will usually lead to a "hands-free" orgasm that starts deep from the inside (what I've termed an anal orgasm). Once these contractions smooth out to a gentler pattern (for a while), I can enjoy the buildup to orgasm IF the waves of contractions don't get too strong too rapidly.
In this specific example, however, I was interested in just how long I could hold on to the enema. I had the balloons inflated as much as I dared without popping them, and the check valve and main valve prevented any backflow into the bag. I was very well sealed by the balloons and I was quite surprised I made it to 45 minutes. Could I hold it 5 minutes more (without orgasing because I'd almost certaily have to expel after that)? And that was the question I asked myself as each 5-minute increment passed as I hung on with the gradually increasing intensity of contractions. Of course there was more to this mental game than "could I hold it?" I used the memory of a high school friend of mine who discovered my enema fetish when we were college students and administered a wonderful enema to me. (That's a very personal story for me and one that I've only partially shared for a variety of reasons. She and I had plans to do more (enemas and whatever else that might have lead to), but that is story of a road not taken. It's not a feeling of regret for not taking that road, but a wondering of "what if?" We lost contact with each other a few years later and she has never attended the high school reunions I've attended. In a private moment, she'd probably ask me if I was still into enemas. But I have no way of knowing for sure or where that conversation might go.)
In the 74th minute, while seated over the toilet, the pressure got too great for the inflated inner balloon and the pressure pushed the inflated balloon out of me for a long initial discharge. Somewhere in the middle of that first discharge, with all the softened or dissolved clumps flowing out of me, I came. It felt so good just letting it all go. The ensuing baking soda rinse enema gave evidence of just how good a job the SSE/glycerin enema did in cleaning me out.
How long can/could I hold smaller volumes? I'm not really sure because the time/volume/solution relationship is not an exact one. But these are the ones I've achieved (and recorded) in my recent enema experiences.