The buzz does last as long as you think it would. I do mine at 830 PM and I'm tired by 1130 12pm. I highly suggest you do them they dexot your whole body.
Do NOT go over board with the amount of coffee and only use light roasted organic ground coffee. I just tried one and a have table spoons and my bum is burning due to the acid in the coffee.
So yes you can serverly damage your large intestine if you use to much or use coffee that is highly acidic.
I would start out with 1 level table spoon full then go from there.