I'm glad you asked this question. While a sauna can be a fun and relaxing pass time, they can be dangerous to your health under certain conditions. This is because saunas can cause hyperthermia, a condition where the bodies temperature rises beyond where it is supposed to. The likelihood of have a health problem in a sauna depends on the temperature, how long you stay in, and if you have any health problems. Only your doctor
who will know your health conditions (if any) can tell you how long you can stay in. In general, you should use common sense. If something doesn't feel right while you in the sauna, you should get out.
On a side note, while the sauna will definitely make you sweat, and bring blood flow to your skin (which can feel good), it absolutely will not help you lose weight. True weight loss
only occurs when you burn more calories than you consume. All weight loss techniques that are successful involve eating less and burning more calories.
I suggest that you schedule an appointment with your primary care physician
. He or she will be able to give you the best advice as to how long you can stay in the sauna with your past medical history in mind. The two of you can also discuss a healthy weight loss strategy that you can start with. Good luck.