A new High Speed Rail Project is in development between Paris and Madrid. By adding the Eurostar from London, it would be possible to get from London to Madrid via Paris is about 8 hours. Actual time may vary if Lyon and Barcelona are added to the route.
Originally planned for 2012--there are delays. Are you surprised?
High Speed Rail
BTW--to give you some idea of the distance between Paris and Madrid: It's basically the same as New York to Chicago or Los Angeles to Albuquerque or Seattle to Sacramento.