How long should the battery go between driving before it does not have enough juice to crank the engine.

I have a 72 280se 4.5 and I am finding that the battery does not hold its charge more than a week. I try to drive the car on a weekly basis to keep the battery charged and drive about 15 miles. The driving route is a mixture of local and highway. The battery was replaced last year and is the correct size CCA rating. Is there a current draw one these cars when they are parked.