I also just had a Miralax cleanout as a coloscopy prep. The same amount, the same schedule. Perhaps because I did cut down my food intake for a full day before, according to my particular doctor's instructions, very little happened. There was almost no osmotic action, and most of the liquid I ingested was excreted through the kidneys. I started at 4:00 PM. I was peeing every few minutes, but had just three "sit down" trips to the bathroom by midnight, none of them with much volume or urgency, and none through the night - there just must not have been much inside. I was concerned enough that I called the doctor's office in the morning, and asked if I could take a full bottle of magnesium citrate as a "chaser". I did, and still, just a minor reaction, no flood. From previous experiences with this agent, I had expected much more. The procedure was a success, and the doctor said that I must have started almost empty, which made the prep so easy and non-eventful. Perhaps I'll try Miralax sometime in the middle of a normal pattern of eating, and see if it unleashes an epic flood, but for me, Miralax was a non-event.
As I have posted before, however, castor oil produced a complete flush of the whole system, from the very beginning of the small intestine, each time resulting in a memorable release. It takes longer for me than for most - empty or full stomach, morning or evening has not mattered; it always takes between 6 and 8 hours. It produces a volume that cannot be held back, and release just happens, starting with an involuntary push and not stopping until the diaper is full. Several waves. The last time, I ate some high fiber cereal & milk just before the action started, and the residual oil in my system sent it through quickly, extending the duration and the volume of the results.