Yes, I did keep a supply of formula-the free samples, which was a total of like six cans.
DD was EBF for her first 3 months. And that was a STRUGGLE. AF returned right away, and she refused to nurse during AF. She lost weight, she was 4 months old before she even weighed 10lbs. We had to give her pumped milk in a bottle, because she simply wasn't taking enough in just from nursing, though we tried for a while.
Then, because of her refusal to nurse during AF (which was regular right away and lasted for over a week) my supply took a pretty big hit. I gave up the struggle at about 5 and a half months old. At that point, she had been sleeping through the night for quite a while (on her own) and I was still waking myself up once or twice a night to pump, and still couldn't get enough nursing and pumping to maintain my supply. (there were also PPD issues involved etc. ) Still, I did have enough in the freezer that she got pumped milk only for another month.
So while we did have that free sample stash, we didn't crack it until she was 6 months old and I was not nursing anymore at that point anyway.
I plan to keep a small supply this time as well, but I am going to pump from the beginning and hope that I don't have the refusal to nurse during AF this time around. I think if I can avoid the AF nursing strike, and start pumping after each session right away (I have to go back to work, so I have to pump anyway) that I can avoid the supply issue and won't need it. If I don't use it, I will just donate it to someone who does need it. Besides, if I am not paying for it anyway, I don't see a point in not keeping it in case I do have the same issues and need to go to formula at some point. The samples are good for like a year so...
ETA: I meant that she was only nursed from the breast for 3 months, then breast and bottles of pumped milk for another 2.5, then just pumped milk from my freezer stash, then formula around 6 months.
ETA2: I always had a bottle of pumped milk ready to go in the fridge if I had to have a family member watching her, so that if they felt like she was hungry, even if she had JUST nursed, they still had something ready to feed-an already prepped bottle takes less work than having to prepare a bottle from an unopened can of powdered formula