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I would say for most cell lines 20 minutes would be okay. If someone is having low viability following a thaw and they froze healthy cells the problem is most likely the thaw. The correct way to thaw animal cells is to bring a cup of 37C water to the freezer. Immediately upon removing the vial from the freezer place the vial in the 37C water than proceed to the water bath to complete the thaw. Some thaw until the last small bit of ice remains, some thaw until the ice is completely gone. Do not immerse the cap of the vial under water. If you do this, I bet your recovery will be better. The damage upon thawing occurs at about -50C so the goal is to transition quickly.