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.
I had a location where we would sell around 40 trails best double salami a week until the manager DIED and the sales dropped to zero! One of the employees insinuated I killed him.
I had an airgas location in Kent WA for quite a while where the manager bought 4-5 bottles of 20oz Diet Coke daily. Pretty slow account overall but super respectful of the equipment.
Everett Canteen is sad to see you're so busy Poplady1!!
Just do it. You're never going to get anyone excited about price increases. They'll kick you out if they don't like it whether you tell them or not. People don't understand vending has expenses just like any other business.