How long to get from London kings cross to London Liverpool street using the London under ground?

From platform to platform about fifteen minutes on the circle (yellow) or hammersmith and city (pink) lines, which run along the same tracks - i.e. when you're on the platform you can get on the first train and it doesn't matter whether it's a circle or hammersmith and city train. I would suggest allowing an extra ten minutes to find your way to the right platform at kings cross, and to get your bearings at Liverpool St station.