The length of time I can enjoy prolonged enemas, either a very long single enema like my Domme exes liked to give me, or multiple enemas held for various times adding up to a long session depends simply on a couple of factors.
1) very sensitive anorectal tissues easily irritated by multiple nozzles, fillings and expulsions,
2) intense enjoyment of insertion, filling, retention and distention sensations for long periods which are also extremely submissively erotic, up to the point where they produce an orgasm. So up until the orgasm, as with multiple edging, the more enema and/or other kink she makes me do, the more it stimulates my libido and it can be enjoyable for many hours. After the orgasm, I largely lose interest in more orgasms or kink stimulation until my refractory period is over in hours to days.
So my Domme exes would use all my kinks and dress me in total enclosure rubber and sneakers and restrain me as her sub while giving me very large and long or multiple enemas, all of which I enjoyed more and more as they kept increasing my need for an orgasm. Often she'd do this for several hours of rubber and enema(s), about 2-4, and occasionally she'd tell me I had to keep doing it overnight so she could wake up and edge me frequently. I don't think we ever went as long as 12 hours, but certainly 6-8 or more. Extremely arousing, enjoyable and erotic.
Rarely, when she felt especially dominant and wanted to push limits, she'd give me an orgasm after several hours of enemas in rubber but then not let me empty and undress and force me to stay covered in rubber and full of enema long enough to begin to get aroused again. She did love watching me submit to her (and my) kinks.
Eric