answer:Probably every 1–2 times that I refill it. Some of mine are also Tupperware, and I know what you mean about them being hard to dry. I usually towel dry, then leave it sitting out for a day or two. If larvae in your flour (or anything else – rice, cereal) are a concern, try wrapping the bag in plastic and freezing it for a couple of days before putting it into your canisters. That should kill any eggs before they hatch, and eliminate the problem before it starts.