my car does take a while to heat up, i start it like 3 mintues before i leave to school and it take about another 5 to heat all the way up. Comapred to my friends car it takes forever
let a few things being checked before you start replacing the heater core...
First of all check the pressure of fluid of the airco
after that check if the airco radiator doesn't freeze when the heater is working..
if it freezes, it can be aircofluid, the temparature sensor or fouling in the aircopipes...
If there's a problem with the airflow... it can be the unit that's controls the airflow or 1 or both fans can be broken...
Small tip: when you turn on the heater push the economy button... it wil cause the airco to bypass the compressor... If the problem is something with the aircoradiator freezing this should help a little bit and it will give u a hint where to look...