I think it depends on how your bowels are to begin with. I was a little constipated. Took the 4 ducalax @ 5:15 pm. Started the miralax solution @ 6:25 pm. I went to the bathroom a couple of times and had small solid pieces. Then about 8:30 I threw up 3 times, so slowed down on how fast I was drinking. Finally things started moving right before 11 pm about the time I finished my solution. I decided I better make extra to make up for what I brought back up. So made another 16 oz with 3 scoops of miralax. Fortunately I had another bottle. So I will probably be up most the night.