IMO, you'd be safe to check in at least 2.5-2 hours prior to departure.
Just a couple weeks ago my Mom and I went to WDW from Toronto Pearson. We were there 3 hours prior but Air Transat ended up having technical difficulties with their computer system. Our flight (supposedly) departed at 6:30pm. We arrived at 2:30pm. We waited until 3pm (3 hours prior) but the check-in desks had their problems and we were told that the earliest they would start would be 4pm. Finally, at about 4:15pm they started check-in. We pre-cleared customs (I think they do this more often on evening flights) and got to our gate by about 5pm and had enough time to grab a bite to eat. Then the clouds came rolling in and there was a severe thunderstorm, causing the airport to close down, meaning no planes could take off or land. Our flight was then delayed from an original departure time of 6:30pm to 8:00pm. Now, let's just add up the amount of hours here.... um, yeah. We spent a total of 5.5 hours in the airport before we even got on the plane.
So you never know... everything could go smoothly and you should check-in with at least 3 hours to spare or you could check in 2 hours prior and still have ample time to twiddle your thumbs at the gate.