VeryGnawty
You might want to look into Tanglewood Wellness Center operated by a man known as Loren Lockman. I believe it is located in Costa Rica.
I've never been to any fasting centers. But, I've followed his YouTube channel for quite some time. He seems fairly knowledgeable about fasting.
I'm not sure how long you are talking about. I believe he does fasts for up to several weeks long.