milk question - when should my mom take frozen bags out so they are thawed for use by the next day?

I am going on a trip this weekend and my mom is staying here with my one month old son. My mom will be bottle feeding him frozen milk that I have pumped in advance. My question is - does anyone know about how long it will take for those bags to thaw? Its only good for 24 hours after it thaws and I just want to make sure my mom doesn't do it too early but give it enough time that it will be mostly thawed. I am thinking the night before the bags are need - I'll have her put them in the refrigerator... any advice here from any nursing moms?