I have found my battery only last about two weeks sometimes too. I had to get a battery maintainer. The reason I think my battery life is so short is that I only put about 20 miles a week on my car. I don't think with using the alarm and only driving it for 20 miles a week is enough to keep the battery charged. The one I use is from Battery Tender.
http://batterytender.com/products/automotive/battery-tender-plus-high-efficiency.html
I plug it into the cigarette lighter/ accessory outlet and run the cord outside the door then shut the car door. I put the charger/maintainer on a brick under the car and run the extension cord to it.
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