If you're starving yourself to death it'll hurt. I've tried it before without eating or drinking up to day five, eventually I gave in and drank some water which made me eat something.
From what I've experienced, you'll experience pain and it'll be a while.
Your mouth being dry and lips being chapped - You want chapstick and a small spray bottle with just enough water to moisten your mouth along with mouthwash.
Then you'll slowly get into a delirious state from dehydration, you'll seek out food and water subconsciously due to the survival instinct.
You'll also suffer from the many symptoms of dehydration which will affect all your organs and your brain, it hurts.
I believe a lot of people who seek this out are elderly/people in hospice where they can have proper medical care and medical-grade pain killers to alleviate the pain.
I personally don't recommend it, but if you're CTB anyway fasting does no harm and actually helps with mental acuity as long as you continue to drink water. If you don't drink water, fasting can take more than a week even without water depending on how healthy you are.