Q&A: Can I Give My Baby Formula Sometimes?
No. Formula changes the types of bacteria that colonize in your baby's intestines. These changes are more dramatic in the early days or weeks of your baby's life and become less so as your baby gets older and eventually transitions to taking solids. Because of this, the later formula is introduced, the better for your baby. Plus, in general it's better to wait and see if your baby actually needs additional supplement before offering it.
Consider starting a freezer stash of pumped breast milk for baby, so you can go as long as possible before — or even avoid — giving her infant formula.