I've never experienced a Far Infrared Sauna , but I understand it's the Best sauna. I've used a steam sauna many times...both while Fasting, and not. Start with 5-minutes, and increase your time as your Body becomes purer and purer. The better your internal condition, the longer you can stay, and will want to stay. It can be very cleansing.
I use a community sauna, and some of our members can't stay more than a few minutes...they get so uncomfortable......WY