hi there
I have a 92 lex es 300 and when i turn on the air conditioning it kicks in and starts working just fine for 5-15 minutes and then it stops blowing cold air and just blows regular air.

any suggestions
thanks

You could need your a/c re-charged....sounds like your low pressure switch is turning the compressor off after running for a short period of time.

It could be that the expansion valve is not working properly. Also inspect first the relay. Prolly a bad AC compressor (http://www.partstrain.com/ShopByDepartment/AC_Compressor).

Bring it to the shop. Sadly, the pressure it requires is something only a shop can take care of... (the re-charge, that is).

Is your temperature rising after turning on the a/c? if that's the case your Lexus radiator (http://www.overnightradiator.com/lexus-radiator.html) probably has a damage that also affect the a/c of the car.

imo, you guys looking for a problem in the wrong place.
clean out your condenser, (it looks just like radiator, and is right in front of the one you put antifreeze in.

there must be sufficiant air flow so it condenses freon, cuz looks like you got the flow restricted, and it shuts of on high pressure, it is either dirt, on condenser, or radiator, i,ve seen many cases where leafs stick to cond, and fvck up airflow,
i doubt you are low on freon, if it was low on freon it either wouldn,t turn on or freeze the evaporator coil..
wash radiator and condenser with a hose (not pressure washer) and than blow water, dirt out with compressed air, try to blow air 90* to the rad. or you will bend fins, and restrict airflow permanently.
it could be defective valve, but in my experience, it almost never fails, especially on lexuses.
i can,t garrantee it is your problem (but most likely it is) , but at least, it worth a try, it won,t cost you anything.