We normaly check-in 30 minutes before departure at St Pancras International, you may want to arrive a bit earlier if you've got lots of luggage or want to do some last minute shopping. The Eurostar terminal is very well sign posted.
At Chessy Marne la Vallée we normaly use the Disney Express service so we get there about 40 minutes before departure to give us time to collect our bags from the Disney Express Luggage Desk.