the rule of thumb as i have always known it is that you arrive at the airport 2 hours before your flight leaves, or 3 hours if it’s an international flight. my flight to seoul departs at 1 am, so i get to ohare on time at 10 pm. getting checked in and through security took me less than 20 minutes, so i’ve got something like 2 hours to kill before boarding starts. luckily i’m flying business class, so i get to chill in the vip lounge drinking free booze and munching on free snacks. (being business class probably is why check in was so quick in the first place. i got to cut in front of all of the suckers flying coach.) 
thank goodness i brought some movies!