My typical rate is a little faster than a quart per minute; this morning's 4 quarts I timed at 4:07, so a little slower than average for me. The bag was at its usual 3 feet above, well, above where the nozzle's inserted, the water was at its usual temperature, and I had voided, as usual, before taking my morning enema, so I don't know the the reason for the slight slowness, and I don't care, it's just one of the vicissitudes of getting an enema.
HOWEVER, I've been taking enemas on a very regular basis for over 40 years, but for someone at the opposite end of the spectrum, for an enema virgin, I would not recommend anything higher or faster what is standard nursing procedure, 18 to 24 inches high and expecting 1 to 3 pints to take 5 to 15 minutes.
I find that all sorts of factors affect the rate of flow including the solution taken, temperature, height, diameter of the hose, position taken in, and, of course, how full the colon is to begin with.