How long can I keep a bottle of formula?

Hey guys, I recently started supplementing our breastfeeding with formula so I’m new to it, and I was wondering how long after mixing a bottle of formula can I still feed it to her if she doesn’t finish the bottle if it’s sitting out?

I posted before how we suffered a huge storm and I’m going on day 4 with no power and no end in sight. So no access to be able to refrigerate anything either and stores are selling out of essentials so I’m trying to conserve what I have.