When your car air conditioning is blowing hot air, this means that refrigerant cannot absorb the heat coming from your car passenger compartment.
This kind of problem is often caused by a refrigerant leak or a clogged condenser.
If you have a steady reduction of cold air from your car air conditioning system, the possibility is that the refrigerant is slowly leaking out of the hose or seal. You will need to have the a/c system recharged with refrigerant first - to have the system checked for any leaks. Then, have those leaks repaired.
Another reason for the gradual reduction of cold air produced by your car air conditioning system is the clogged condenser but this is only a contributing factor.
To correct the problem, you must first check the condenser for dirt, debris or bug that is building up outside the car condenser, this is the part that is like a mini-radiator mounted in front of your car radiator. Try to clean this with a soft brush also, spraying with pressurized water will do. This may improve your car air conditioning problem.