As needed any day of the week. You shouldn't be opening any machine unless it meets your cash pull criteria or you're standing right next to it. Of course that means you'll have to properly merchandise every machine so the pulls you want are realistic. I liked to keep my snack and soda machines par set together so if the soda was the money maker and the snack was less revenue I'd stock them both to however many days the soda could last with the given selections based on 3-6 weeks historical data. Results in less spoils and faster services.