On startup , a natural gas furnace gas heating system has cold air blowing out the registers why?

Cold air in the ducts being displaced by warm air,

How long does the cold air blow out of the registers? Does it eventually get to be warm air?
When you turn the thermostat to heat does it turn the gas burners on. Is this a new heater that you are using for the first time. If it is there is a possibility the you have the wiring wrong, the white wire in the furnace must be connected to the neutral and the black connected to the black hot wire. Otherwise the furnace will only blow and do nothing else.