2002 Nissan Altima Heater blows cool for a long time before

Try warming the Vehicle, then carefully feel the heater hoses, are they both hot? If one is cooler than the other, You need to flush the core, or do a replacement job on the heater core. Also if the system has a heat control valve on the heater hose, make sure it is working! Also check the thermostat, and the temperature blend door, thats under the dash, in the heater plenum. You can flush this yourself, buy the prestone flush kit and install the tee in the heater return line. Disconnect the heater hose at the waterpump, clamp the hose in back of the tee you installed and back flush till it runs clear.20 mins or so.