I agree with Fred01CA. Castor oil is quite unpredictable. In my experience, it has taken anywhere from just under one hour to over nine hours. I was starting to wonder if anything was going to happen. We are all different, but for me, there is a long build-up of mild pressure, and repeated gurgles, but nothing overwhelming. Over time, however, the ability to clench decreases - that sphincter gets tired, like any muscle does. At the same time, volume and pressure both build, and begin to come in waves, with less time between urges. The muscles throughout the intestines contract with more and more insistence, until there is no way to resist. At the same time, water is drawn into the intestines, so it becomes more and more liquid the longer you hold it in. After expulsion starts, the urge to expel persists even when the rectum is empty, the anal equivalent of dry heaves. There may be a long time where you feel more and more full, but with few contractions or real urgency. Don't be disappointed - it doesn't last. The moment will come when you are overwhelmed, and you will never know exactly when you will clench, and at the same time, push involuntarily. The push will win, decisively and suddenly.
There are many threads on that discuss castor oil, how to mask the taste, etc. Splitting the dose into 4 equal parts, taken 10 - 15 minutes apart, helps some people achieve a faster and more spectacular effect. Most recommend a total dose between 2 - 4 ounces; I usually take around 3 oz, and it has never failed. I can usually tolerate taking it directly from the bottle, but once tried a fellow Zity member's recommendation to whisk it into whole milk (the fat helps the effect) with a lot of chocolate syrup. I could hardly tell it was there when I took it. It definitely made ingesting it much more pleasant. I could certainly tell that I had taken it a few hours later ...