When you boil an egg, a physical change occurs that causes it to harden. The time it takes to hard boil an egg depends on the size of the egg. Give the egg time to reach room temperature, about 12 to 15 minutes. Place the eggs in a pan of water and give the water time to boil. It should take eight to 10 minutes for the water to begin to boil, less time if you cover the pan with a lid. You can also add salt to the water to encourage the water to boil faster.