Assuming you are talking about a traditional sauna, there is no set answer. It depends on how long you can tolerate the heat. The key is to make sure you are sweating profusely. That is when your body will start to detoxify itself. So, if you can only handle sitting in the sauna for 15 minutes before you start feeling faint, then that is fine. You will still sweat and your body will still benefit from it. Just make sure to drink plenty of fluids afterwards so you do not become dehydrated.