how long should a heater core flush be good for?

A clogged heater core will not cause the engine to overheat. It can cause the heater to not blow hot air. Check your coolant level. If it's low, refill it, bleed the cooling system to get any air out of the system and check for leaks. If you have the 3.8L V6 look for coolant pooling on top of the engine and pull the oil dipstick. If you have white milky goop on the dipstick and/or coolant on top of the engine, the intake manifold gaskets are leaking. It's NOT going to be the head gaskets! The plastic upper intake manifold should be replaced, don't reuse the old one. They always crack. If you have the 3.1L V6 and you have white goop on the oil dipstick, it's going to be the head gaskets. Other causes of overheating are bad water pump, radiator cooling fans not working, clogged radiator and/or clogged cooling system passages in the engine, air in the cooling system, and as mentioned, low coolant level. Hope that helps! Jim