The amount of water that needs to be placed in the pot depends on how dry the air is in your room. Homes that have central heat tend to be drier as the home's unit regulates the temperature through forced air. A large, stainless steel pot filled with 1 to 2 gallons of water is ideal if you wish to leave the water boiling throughout the day.
The stove's dial should be set to high initially to boil the water, followed by turning down the dial to a medium setting once the water begins to boil. Water that continuously boils at a high temperature evaporates faster than a low-temperature boil and therefore requires frequently refilling to prevent the pot from burning.
Your stove doesn't have to be the sole device to boil water as steam radiators create a similar boiling effect. While the evaporation rate is much slower, steam radiators warm up the pot of water while actively dispensing moisture into the air. The benefit of steam radiator use is that the water can be left unattended, whereas a boiling pot of water on the stove should never be left unattended due to the risk of fire.
Always use a clean pot to boil water, and replace the remaining water after each use to reduce the spread of allergens in the air. Airborne allergens settle in the water and are redistributed in the air, which can cause allergy flareups if they are not promptly cleaned from the standing water.