I have a garage queen that would sit for about a week before taking it out to drive and the battery doesn't drain. I too just invested in a battery tender due to it now sitting for much longer periods of time and it wont be driven very far distances anymore. Do you drive the vehicle often? Sitting and short drives also tend to shorten battery life. Posted our tech article about the battery below if you want some additional info.
Porsche Boxster Battery Disconnect Switch / Battery Buddy Installation - 986 / 987 (1997-08) - Pelican Parts Technical Article