It takes about an hour by train from Farnham to London Waterloo
UK - How Long Does The Train From Farnham To London Waterloo Take?
It takes 55 minutes, or slightly less at certain times. The London station is Waterloo and there is a train roughly every half hour at busy times. If you click here you can find information about train times and prices, and even buy your ticket online, or look up different trains.