It never ceases to amaze me how unpredictable oral psychedelics are.
I've recently started taking niacin 500mg, which is not a psychedelic, but I'm mentioning it here, because its flushing effect is unmistakable and makes it a good model for studying oral absorption.
I read the effects come in 30 minutes. When I took it for the first time, I waited an hour, didn't feel anything and thought "Ok, if I haven't felt it by now, I'm not going to feel it. Maybe I'm just not a flusher." I forgot about it and carried on with my stuff.
2.5 hours after dosing, I went to the kitchen to make a smoothie. One avocado and half a banana. Before I finished the cup, which took maybe 2 minutes, I started feeling heat on my scalp. "Woah, what's going on?" The overwhelming heat spread to my neck, arms and legs and then I realized it was the niacin from 2.5 hours prior which I had forgotten about.
Now I know that when I take a niacin cap, food will kick it in in no time, but until I eat something, I likely won't feel it at all. If I take it after a meal though, it comes on on its own, but it's less intense.