The amount of time a Thermos will keep boiling water hot varies based on the container and the manner in which the container is prepared. According to one manufacturer, the proper way to prepare a container for hot liquids is to fill the container with warm water, place the lid on the container and allow the container to sit for 10 to 15 minutes before use. This process may minimise breakage, as well. Most containers will keep the water warm for up to 12 hours. However, the water will begin to cool before that time. Some containers are rated for up to 24 hours.
Keeping the Water Safe to Drink
It is difficult to ascertain how long boiling water will stay safe to drink once placed in a Thermos. In general, fresh water will only last about a week. Theoretically, sterilised water in a sterile container will last indefinitely. The process of boiling water, however, does not necessarily purify water. According to at least one scientist, boiling water kills most bacteria but does not kill all bacteria.
Individuals that wish to keep water hot will likely be able to leave the water in a Thermos for at least 12 hours. This is contingent on the container being properly prepared ahead of time. If the purpose is simply to keep the water safe to drink, it should be kept for not more than one week.