Defrosted BM Question

Samantha
I've headed back to work this week and my hubby was curious whether her can split up BM after it's been defrosted in warm water. 
For example, I froze 4 oz of milk, but she may only need 2 oz at that time so he would give her the 2 oz and save the other 2 oz in a different bottle. Within what time period would he need to give the other 2 oz?