We use the Credo Thermo coolers too. I have 10 liners (units) for 5 coolers - you will need the extras to keep up with the rotation. We freeze the units for at least 8 hours in a -30C freezer to recharge them. I keep 4 units in the refrigerator at all times. After you remove them from the freezer, they will take 2-3 hours in the refrigerator to come to the proper temp (this can be determined to be when there is no frost left on the outside of the unit), but since I am rotating 4 of them at all times in the frig, we always have one ready to go. (In moist climates, they sweat a lot - place them on towels to soak up the water.) If they have been in the frig over 24-48 hours, you can always rotate some back to the freezer and replace them with ones from the freezer (this happens after the weekend when they don't rotate out as fast). Hope this helps - they are small coolers and will take only 2-4 units, but the RNs love them. The Thermo-Safe coolers are much heavier and require 3 ice blocks to keep them at temp - the RNs hated carrying them. Make sure you label the Credo lids - "Don't lose this Lid" and label the back of the lid with "Return to Blood Bank" it will help you keep up with the lids - if they are lost, the unit has to be replaced - expensive.