Nissan Repair: A/C repair, o ring, freon

Question
The A/C in my '96 Maxima did not blow cold air.  The service replaced the freon and put some die into the sysytem.  After a couple of months it would not blow cold air again.  It was winter so I waited until summer and called them.  They say they have to start all over again.  Is this reasonable?  I saw some green dye near the tube joints on top.  Could this be the leak?  What cost am I looking at here?  Please advise.  Thanks.  

Answer
Kam,

Yes,

They have to find the leak and the only way to do it is to charge the system with a dye and then trace the source of the leak.  If they charge it again it will probably last a few months so be sure to take it back and find the leak.  It could be as simple as an o ring or as complex as a condenser or evaporator.  Only way to tell is to charge it and find the source of the leak.