I usually don't like to let mine go more than 10-12 days without checking it.
The problem you have is this: You don't want to get them in a pattern of visiting your feeder, then have the battery run down. They will immediately know there is no food there, so they will move on to where there is feed. So then, you put a new battery in and you have to start all over. So, I would do my best to never let it quit, or you'll be re-starting all over again.
Hope that helps! -Let us know if you bag something!