what do you do? help?

I just started enfamil formula for my baby and am also breastfeeding but when am not home, my husband gives him the formula. And I was wondering if baby doesn't finish the milk, do we throw it? Or should we put it in the fridge? How long can it be outside? When can we not give it to him? I have so many questions. Please answer me if u are formula feeding your baby too or if you know the answers? Thank you