I can't remember exactly how long it was... maybe 4 1/2 hours? I found it a pleasant experience although it's frustrating in England because the rail tracks are rubbish and therefore the train has to go fairly slow compared to in France. Enjoy!
It takes about 2 hours 45 mins. When I went last week, it was very comfortable, quiet, with food etc. available, but the check-in procedure was complete chaos in both directions.