It takes five to 10 minutes to boil 6 to 8 cups of water. The water, pot and stove temperatures; the water's salinity; the pot's volume, surface area and conductivity; the altitude; and whether or not a lid is in place affect the amount of time needed to boil water.
Hot water boils sooner than cold water. Due to the thin air and lower air pressure, water boiled at high altitudes boils more quickly than water boiled at lower altitudes. Water also boils faster when using conductive copper or aluminum pots with the lid on to trap heat. Salty water and water boiled in larger pots take longer to boil because it takes more energy to vaporize the salt and there's more water to bring to boil in a larger pot.