The amount of time you can spend in cold water without suffering from hypothermia is determined by the water temperature, your body size and shape, your level of cold adaptation, and your experience, among many other factors.
Therefore, it’s best to start with safe, short dips of 30 seconds to learn what your limits are. If you begin to feel uncomfortable or you start to shiver, listen to your body, get out, and slowly warm up by walking around.
If you like, you can check the water temperature before entering the bath to ensure that it’s at a temperature that’s suitable for your experience with cold exposure.