Dragons could easily go for half a moon without food, especially if they didn't spend a lot of time skyborne, as the ambient magic was plentiful in Winterfell. Which was godsent because now Stormstrider and Winter could eat as much as a thousand men in meat for a day. Winterfell would struggle to meet that increasing demand despite all the new large herds of cattle that were specially bred to feed the dragons. Eating that much once every three or four days was far more manageable.