How hard should water based polyurethane get?
Assuming 70°F, 7 days to cure. You stated your temp has been 40°-60° and did not state humidity. There is nothing wrong with the product, you have used it outside of parameters recommended by the manufacturer (see text below). Take it into a temperature and humidity controlled space and hopefully it will cure hard.


  Dry times are based on 70°F and 50% relative humidity.
  Dry times may be affected by temperature, humidity and
  ventilation. Allow more time at cooler temperatures. Triple
  Thick Polyurethane will be dry to touch in approximately 2
  hours and can be recoated in 2 hours. Allow 48 hours cure
  before light use of the project. Allow 7 days for full cure.