I have never had to fill them all the way up and I have been away for up 10 days. If your ladies are young enough five gallons for each four pots will probably be more than enough.
Must be nice to have someone you trust to check on them for you. I spent 10 days in Japan with a tent full and I worried every day about them. Next big trip I’m putting a camera that I can access remotely and that way I will at least know what I’m coming home to or if I should just stay out of country if I saw the police cleaning out my grow. So far, each time I go away the ladies do better without me, I wonder what they are trying to tell me.