From what I've heard from my friends, opened formula powder can only be kept for 30 days (upon opening). The most they have tried was to have it for another 3 to 4 days beyond the 30 days. If I'm not wrong, the nutritional value of the powder may change once opened. Hence, it is probably better to keep to the guideline (usually stated on the can itself). Also, freezing of powder is not recommended as it may change the physical properties of the formula. Perhaps you could opt for a smaller can if you are not using that much.
You need to discard the formula milk after 2 months once opened up. Unless you didn't open the cover very frequently then can last until 3 months. It is best to discard as bacteria will grow inside due to the humid weather in Singapore and it is best not to risk your baby from getting stomach upset.
so far for my kids, when they r using formula, i leave it on the table, away from sunlight, and they can use about 2-3 months. this is in particular for my eldest gal of age 8 as i will normally buy the 1.8kg tin and she drinks only twice a day.
One month and discard, whether finish or not. It's also stated on the tin.
thanks