Did the indie who charged it 6 weeks ago tell you that he added dye to help diagnose a leak? That's a common approach that even a DIYer can do by getting a can of R134 with dye at any autoparts store, and add to the system. If you can add a can to a system with too little refrigerant to trigger the pressure sensor, the sytem will work again for awhile, which will tell you something.

If he did put dye in it, you can look for the leak using a UV light and maybe goggles. But leaks can be in hard to find places (evaporator, back of condenser).

If it did leak (which is a safe assumption), and it doesn't have dye in it, there may still be oil visible where your problem is (the ac system circulates oil with the refrigerant, and they can leak out together).

__________________
1996 C280 289K Traded
1997 E420 167K Traded
2001 S430 240K Traded
2010 E550 4matic 80K
2000 GMC Jimmy 198K Gone to Boneyard
2003 Camry LE 196K
2011 Mazda3 i Sport 31K