Most healthy adults are comfortable for 10 to 20 minutes. If you are new to it, start with five to ten and let how you feel set the pace, not the clock. Cool down for a few minutes first so your heart rate comes down, drink water before you go in and again when you come out, and step out right away if you feel dizzy, lightheaded, or unwell.
Most people use the sauna after training instead of before, so the heat does not take the edge off the workout.