I am new to the board (diagnosed with AS in 2005) and want to thank everyone that posts questions/answers. I thought I would tell you of my personal experience with infrared saunas....
I have an infrared sauna in my home. I do enjoy using it and believe the detoxing effects are wonderful. It also helps me relax when my muscles get overly tight. I do, however, find that when my joint inflammation is flaring, heat actually makes my joints hurt worse. Thus, I have to pick and choose when to use it.
The length of time and temperature really depends upon the sauna you have. The first one I owned, retained the heat really well (less windows/glass) Generally, 20 minutes at around 110-114 degrees was my limit. My current sauna has a lot more glass. It has to reach 130 degrees before I begin to sweat and I can easily stay in it for 30 minutes or more.
(If you decide to purchase an infrared sauna, make sure you buy one with low EMF readings. Some of the lower cost saunas have high EMF readings, which could potentially be harmful to you.)