what kind of battery are u using? what psu?
because you can leave a car on s3 sleep a lot longer than you might think. Mine is setup for voltage instead of a timer, so if my battery's voltage drop below 12.3V (or whatever i like) the psu will hard shutdown. And I've left my car for 2 - 3 days without hard shutdown happening.
I drive my car everyday so it has to stay on s3 at least 12h/day which has never been a problem.