Depending on the temperature of the water, 10-15 minutes is a good amount of time in a hot spring. That said, we often soak much longer – getting in and out of the water as needed to keep a comfortable body temperature. I am not a medical professional and am sharing my experience, not advice, but I listen to my body and get out when it feels like enough.
Drink water (you can get dehydrated in hot springs!) and if you feel lightheaded or dizzy, get out carefully.