The refrigerant will then passes to a part called "condenser" this part of your car air conditioning system is the one responsible for removing heat which the refrigerant absorbed from your car passenger compartment.
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 often cause 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. Another reason for the gradual reduction of cold air produce 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 improved your car air conditioning problem.
If it will not improve your problem then you will need to take your car to a reputable shop specialized in car air conditioning and ask the mechanic to locate the leak and repair, take note that the cause of this kind of problem is mainly a refrigerant leak and the clogged condenser like I have mentioned on the latter is only a contributing factor, which means that you need to "charge" your car air conditioning with an amount of refrigerant. Also, you have to know what type of refrigerant your car uses.
Now a day's all new car model uses an environment friendly R134a refrigerants which is cheaper than the formerly use R12 also known as "Freon" which is banned for being not an ozone friendly.
If your car still uses R12 refrigerants, expect it to be extremely expensive due to its diminishing supply, it would be better to ask your mechanic to convert your car air conditioning system to the one that is cheaper and environment friendly refrigerants like the R134a.