The trick is to start at the end of your limbs and move slowly inward until you think you had enough, and definitely not so fast you start to pant: The calmer you are, giving time for the body to switch to the change, the more you'll be able to take, so take it slow. Also don't feel obliged to use only the cold tap, especially in winter that can be rather extreme. Just up to elbows and knees more often than not get you that nice metabolism boost and that's a perfect pre-coffee, OTOH some days are torso days and even others are head days.