A leak will certainly cause the charge to disappear pretty quickly. There are a number of places that can develop leaks. The compressors are built in sections and can leak between. O rings can perish and leak. Hoses can develop pin holes or breaks at joints and crimp points. Condensors, evaporators, driers, accumulators--all can leak. Aside from that, the o-ringed connections are not sealed by tightening them. Rather the system pressure seals them. As the o-rings get old, it's not unusual for gas to escape. I've had systems last as much as five years, but not past that...yet. I've attempted re-charging systems several times with different products. Each time I swear this will be the time I'm succesful, but something always nixes it, and I have a vacuum pump and all the proper stuff. Perhaps I'm just AC challenged. I have to service a system about every two months and I have at this point given up, deciding it's just easier and more cost effective to have a shop do it. Here in CA, land of the restricted everything, it costs me 180 to 200 to have a system evacuated, flushed (you must change the oil when you change to the new style R134a refrigerant), fittings changed, pressure tested and charged. If the refrigerant is worth about $50 of that, a drier another 25-35, then I figure haivng someone save me the headache for about $120 bucks is a bargain. If the car needs a compressor, hoses, valves, I do that. They do the rest. Once a system is converted I will not hesitate to buy a can of R134a and charge it. However it's not practical for me time-wise to do the conversion. That said, there are now conversion kits that include "conversion oil", which treats all the seals and such to make them compatible with the new oil (supposedly). They still recommend you evacuate the system first though. Jury still out on that.

The 700 FAQ has great info on charging/converting etc. if you are still inclined to tackle it yourself.

Good luck!