Chevrolet: HEATER CORES/ HEATING, ford escort wagon, upper radiator hose

Question
Hi my name is James, My girlfriend has a 97 Ford Escort Wagon with a 2.0L 4cyl. I was wondering how long does it take to remove and install a heater core in this junkbox?
Cause I recently replaced her thermostat and the car still has no cabin heat! I can't get at the heater core hoses by the fire wall under the hood,because there is to many things in the way and I can't get my hands down there to check the hoses for temp. I squeezed the upper radiator hose with both hands and i don't feel any pressure or crunching! When the car is warmed up I turn the temp. knob all the way to hot and fan on high but then the warm air fades to cool air it keeps changing back and forth.The upper radiator hose is warm,And the air isn't blowing out as fast as what it should,is there a screen under the dash somewhere that might be clogged preventing the air to blow out faster if so where?And how long does it take to change the heater core in this junkbox as she calls it?She wants the heat in the cabin to be nice a warm. Cause she can drive 20min. with the temp. knob on hot and the fan on high and the inside of the cabin isn't even nice and toasty!I think the heater core might be plugged or it's the blend door!I have some auto knowledge but it is puzzling me!I need to get it working for her bfore it gets to cold out here! PLEASE HELP ME!!!! thank you  

Answer
James;
I would look to do the heater core the LAST thing, does it have a heat cont. valve on one of the heater hoses, if yes than it could be bad  if the inside of the car does not have a wet floor than the heater core might be fine  when hot are both heater hoses hot or only one,if one than the heat. valve or thermostat is bad, and how is the red. cap.  you could also get a pressure test on the rad. it will tell you if you have any leaks.  ok Merry Christmas