How frequently should you winter swim, and how long should you stay in?
The stages of cold water immersion go like this:
fear and dread
trying not to think as you get in
cold water shock
absolutely loving it
getting really cold
Steps 1-4 are necessary, and add to the afterglow because you feel super-human for having conquered a fear. Steps 5-7 are the stages that your body goes through, and how it becomes adapted to cold water. Steps 8-10 are to be avoided by getting out before you get there.
How long it too long?
It sounds ridiculous to say 'get out when you start to feel really cold'. But as you become an experienced winter swimming, you do get to know when you've had enough. Before you get to that stage, it's worth staying in for up to two minutes per degree of water temperature (centigrade). So, if your thermometer reads seven degrees centigrade, fourteen minutes or less is good amount of time to cap your swim.
But I feel amazing!
Getting out when you're still at step seven, so still absolutely loving it, is ideal. This is because your body will continue to cool after you get out of the water. Known as the afterdrop, as you warm up after getting out of cold water, the cooler blood in your skin and extremities starts to mix with the warm blood that your body has cleverly shunted to your core. That means that your core temperature drops, usually triggering a shiver response deep inside you.
If you stay in for too long, the afterdrop is deeper. Considering that your normal body temperature is around 37 degrees and hypothermia happens when your core temperature is 35 degrees, you don't actually have much room for error.