Each tablet of choline has about 500mg of choline in it and for the test you take 3 the night before - so that's 1500mg. Just to give you some idea of how much choline that is, one serving of ground beef has 85mg of choline. So that's like eating 15 servings of ground beef the night before the test.
In your case you may notice it gradually because a normal choline diet doesn't include 1500mg a day.
For me I notice changes in my body very quickly. For example, when I drink coffee - a few hours later I can smell the coffee smell in my urine, so I know it's making the rest of my body smell. I'm not sure if I have TMAU or not but if I don't I know I have some other metabolic disorder which makes me unable to metabloize certain chemicals like the ones found in coffee, aspirin, alcohol, etc.