If you're in 40 F water and cannot get out of the water then you have around 30 to 60 minutes before hypothermia sets in and you become unconscious.
You will likely die within 90 to 120 minutes after the hypothermia sets in and you become unconscious but sometimes it can happen sooner.
Some people go in the water after there dogs which they should never do but I can understand trying to save a family member or kid but dogs or other animals you should never venture into the water after.
If the water is too cold then don't go swimming in it and never swim, skate or walk or any ice that you're not sure how thick it is.