You'll know you had enough, generally, within minutes of drinking, if you feel subtle waves of either warmth or cold, tingles (especially in the forehead), Caapi does announce her presence when you learn to listen. I would avoid food ~4 hours before journeying so the stomach is empty (I would avoid grains, animal products, anything that considerably ties up the digestive process) so you absorb the medicine more efficiently.
Generally, I find the tea hits very quickly, but I have had the rare occasion where it's taken 2 hours to hit. Signs of Caapi intoxication are body temperature fluctuations, tremors, visual distortions (lots of tracers, sensitivity to light), dreamy feelings. It stones the CNS. Sometimes it might feel like your spirit is going to rocket out through the top of your skull. In the beginning, I often see snakes and light geometric patterns. There is definitely an unmistakable *shift,* so when you start to feel shifted, chances are things are going to work.
Everybody is different. I had a friend once who began experiencing visuals, purged, and then the journey was aborted. But lots of these things also operate on the medicine's agenda. Especially in the beginning, it will take a few tries to get it right, but it is absolutely worth doing the work for--the rewards are immeasurable.
Personally, I like to reduce the heck out of my brew (1 kilo down to 4oz), decant, and then mix the appropriate amount into a tall glass of chocolate almond milk and drink slowly. I find knocking back shots results in a much heavier body load, a shock to the body--but sipping a cup slowly, over 30min-1hr, listening to my body and feeling things out, really takes the edge of the initial shock. It's also impossible to over or underdose that way. It's a gentle way to slide into a visionary state (and much fun to play some music, rattles, guitars, etc., while it slowly comes on).
It's all trial and error! The body learns quickly and the medicine guides... so have faith.
Not sure if I said this already, but coating the mouth with something like honey/maple syrup/agave also REALLY helped me get it down when I was taking shots. If I could keep the brew from directly hitting my tongue, it was much better.
Some things will come easy, some will be a test