does a tub of formula last? i bought one, thinking it would be more economical than opening 2 cartons a day when of each one she only eats 100ml- but just noticed it says discard the tin after 4 weeks. at the moment im making one or two 4-scoop feeds a day, i might increase this to 3 4-scoop feeds in a couple of weeks if i drop the morning breast-feed, but im not switching the nite-feeds (i cant be arsed making up formula at 3am when i got milk in my chest!) so at the most ill be using 12 scoops a day by mid january. am i gonna be wasting loads? is it safe in reality to keep powder longer than 4 weeks?