How long can I leave mixed formula out in room temperature before my baby drinks it?

Depends on situation. If a kid has started to feed on a bottle & you recapped it for later, i like to toss it after 4 -6 hours at room temperature. If you pulled it out of the refrigerator and it sat out at room temperature, it is good for the whole day. This assumes you clean your bottles like you do regular dishes & don't reuse before cleaning.