The duration for which ice can last in a thermos depends on several factors, including the quality and insulation properties of the thermos, the initial temperature of the ice, and the ambient temperature outside the thermos.
A well-insulated thermos can typically keep ice frozen for around 24 to 48 hours. However, it's important to note that this is just a rough estimate, and the actual duration may vary. Factors such as the size and shape of the ice, the amount of air space in the thermos, and the frequency of opening the thermos can also affect how long the ice lasts.
Additionally, some high-quality thermoses may have enhanced insulation capabilities, which can extend the ice retention time. It's always a good idea to check the manufacturer's guidelines or product specifications for more accurate information regarding the specific thermos you are using.