Super-slow flow rates are indeed among the fun e-activities in my repertoire. I find that enjoying them requires a wide-open schedule, where there is absolutely no time-pressure to finish before someone comes home.
I use Susie's method of two clamps, one for on/off and a minutely adjustable trickle clamp to adjust flow rate. Most of the major e-suppliers have the trickle clamps available for purchase. The slow dribble flow rate is frustratingly difficult to calibrate with any precision, but it can be approximated. I remember once spending much of an afternoon hanging the full bag from the shower rod and timing how long it took to empty out into the tub, only to realize later that the flow rate was dramatically affected by "back-pressure" when the noz was inserted into me.
The "head" of the water, the height the bag is hung above the anus, seems to be a greater factor when dribbling than when flow is unrestricted. I find that the flow rate can be estimated better when the bag, and hence the water pressure, is higher. There will be a decrease in flow rate as the bag empties and the head decreases. Please do remember that the head determines the water pressure in the intestine regardless of how slow the flow rate may be.
I set the clamp and check the flow rate before insertion. I usually seek to have the output of the noz just at the point where individual drips become a tiny steady stream, which results in around fifteen or twenty minutes to empty a two quart bag, or a half-hour or more to empty my big Klystra 4Q bag.
Lying there on the bed requires some self discipline when things are happening that slowly. Having something kinky to do, like watching pretty ladies giving each other enemas on a laptop, is a big help! It's a fine line psychologically between wanting some distraction and enjoying the sensations in real-time; I find I enjoy kinky distraction while taking very slow enemas.
Hugs,
Diane