4 mins or so. First 2/3 mins would be used to wash hair etc… then tail off into a cold shower for last 3 or 4 mins. Once you get the breathing right it’seems grand, some initial shock though.
Did you not say that your made a fortune put of the last two fads?!! I generally finish most showers with 2 minutes cold anyway, thoroughly invigorating.
I finish every shower with the last minute at as cold as can be. Being doing this for nearly 15 years I’d say. Can’t imagine not doing it particularly in the morning.
Ideally you’d jump into the sea every morning and have the same effect along with the benefits of the sea water but the cold shower is a great compromise.