Discard after 1 month - do we HAVE to?

[Uno-Mom]

My dd gets formula mixed into her solids and sometimes we add an ounce or two to a breastmilk bottle out of necessity.

The powdered formula package says to discard one month after opening ... but she doesn't use a whole big container full in a month!

What expires about the formula after it's been open that long? Is it important to observe that to the letter, or can we stretch it to 5 or 6 weeks?

I guess we could buy a small container, but it's so much more expenseive that way!