1998 Dodge Charger A/C

I have a 98 Dodge Caravan, last summer we had to add a can of freon, a few months later we had to add another. This spring we have gone from adding more weekly to daily. I has a strange smell that comes from the vents. Once you add the freon it works very well, but just not seems to last very long. Is this something more than just a leak in the line?