Short term human milk storage guidelines were discussed in a recent paper:
Hamosh M, et al: Breastfeeding and the working mother: effect of time and
temperature of short term storage on proteolysis, lipolysis, and bacterial
growth in milk. Pediatrics 1996; 97:492-498. All of the guidelines below are
for healthy full term infants.

38 degrees C (100 degrees F) safe storage less than 4 hours
25 degrees C (77 degrees F) safe storage for 4 hours
15 degrees C (59 degrees F) safe storage 24 hours. This was in a styrofoam
chest with blue ice freezer pack

We have already looked at data for longer storage:

4-8 days in a refrigerator
2 weeks frozen at greater than 0 degrees
6 months frozen in a deep freeze at less than 0 degrees

I get calls all the time from day care providers and working mothers asking
about how long milk can be stored. I have used these guidelines with good
results.

Marsha Walker
Weston, Massachusetts