Similac supplement bottle users..

Ive been topping my bottles with pumped breastmilk with similac for supplementing pre-made bottles, and usually dont even ise a whole one at a time, there are no instructions on the box about if u have to refrigerate after opening or if u can continue to leave it out? Ive been putting them in the refrigerator but then i have to warm them 2 hrs later when i use the rest.. ((Ps i know your supposed to throw out if they drank out of it))