Get a refrigerant charge from an A/C shop - and ask them to include leak testing dye in the charge.
If they tell you that the car is fully charged already - then it's likely a mechanical or electrical problem (compressor, clutch, relay, A/C board, etc.)
If not - then you probably have a leak. When the system loses its charge, take it back to the shop and they can check to see where the leak came from - pinpointing your problem.
Hope this helps. I'm doing this same process right now. I fear that my leak is from the evaporator. There go the vacation savings!