AC works good for about 30 minutes but then the airflow gradually reduces and the air gets warm. refrigerant level is good, inside passenger cabin filter is good.
I need more information on your car like the year, engine size and anything important that can help me diagnosis your problem.
One thing you can check is to see if all of your engine grounds are clean and tight, a loose ground can cause this type of problem.
Get back to me with that information when you can.
This may be an evaporator freeze-up. There's a temperature sensor at the evaporator. When evaporator temperature approaches freezing, the PCM will turn off the compressor until the temperature rises.
When this switch is not working, or the temperature data is inaccurate, the condensation on the evaporator may freeze. This will block air flow and eventually prevent cooling.