Combi mixed feeding help!

I mixed 2 oz of freshly pumped breast milk with 2 ounces of aptamil ready to feed formula milk. My daughter has had 1.5 oz only… can I put it in the fridge? If yes, can I then re heat it and make it warm? If not, how long do I have to feed her before I need to discard it? For info- my daughter is 15 weeks and exclusively breastfed. Trying to get her used to the bottle as I am introducing formula with the hope to stop breastfeeding and move fully to formula by 6 months