Feeding once every 6 or 7 days is very, very tricky. You'll need to have 12 kilograms capacity for that feeder, that is 20 lbs, for those on the English standard. I wouldn't trust anything less would be enough for a week, for 12 adult hens. My questions would be this.
1. What feeding device do you have that would administer feed, that would hold 12 kilos of feed?
2. Are you confident that the watering devices can last for up to 3 days? I would not be comfortable with anything less than that capacity.
3. Finally, who's collecting the eggs and how often?