Ford radio only works for an hour at a time?

Since having to enter the radio code after changing the battery the radio/cd player only works for one hour at a time even with the car running.
If the unit is on and you turn off the ignition the radio will stay on until the hour is up or you turn it off on the knob. Before coding it the ignition turning on and off would also turn the unit on and off. Not too great a deal as you just turn radio back on for another hour on long journeys, just annoying as to why it is doing this??
Many thanks