These estimates are always highly approximate. As an alternative, perhaps slightly more optimistic, see Prof. Popsicle's (Gordon Geisbrecht) chart at
http://www.coldwaterbootcamp.com/pages/1_10_60v2.html. Note that this chart is in degrees C so you will need to convert to F. Also note that immersion hypothermia is one of the few areas where it is advantageous to be a bit overweight!

Note that these survival times assume you have floatation. Without a PFD, you rapidly loose strength and coordination, and it becomes inceasingly difficult to stay afloat. In most cases without a PFD or other floatation you will drown before dying from hypothermia.

A few years ago we had a remarkable case here in Alaska showing just how valuable a PFD can be even in very cold water:
Kenai girl credited with saving friends in Tustumena Lake accident. The girls were in the water for about 2 hours, and remained conscious and able to move towards shore, with water temperature of about 40-45 F (4-7 C).