The manual states it will go 105k at first and then change it every 45k after that. The reason for this is that the dealerships will fill the system back up w/ tap water. The minerals in the tap water reduce the life. If you use distilled water you should be OK to go well past 45k.
Ford's recent mailing pretty much states to test the coolant strength every 45k (or every 15k optional). They state to add supplemental coolant additive (VC-8) if the nitrite strength is between 300 ppm and 800 ppm. They state to flush the coolant if the nitirite strength is below 300 ppm.
I believe the main issue is that combustion gasses getting into the coolant (leaking head gaskets and EGR cooler leaks) deplete the strength of the coolant.