Should not store in the fridge, else the powder will start to clamp. Normally, less than a month will be finish. My infant is 4 mths old and took him 2 weeks to finish a tin of S26. If you're talking about untouched formula milk then it can be kept in the fridge for up to 24 hrs. If baby has already drink a bit then need to finish within an hour and should throw away if baby does not drink after that period.
Formula in powder form should NOT be stored in a fridge but just in a cool, dry place, like on the kitchen counter with lid closed. Once open, use within a month. If u're talking about a prepared bottle of formula milk, if baby hasn't drunk, can keep at the back of the fridge for 24 hours.