If you don't have bags and are checked in online, you can totally show up 20 minutes before a flight (just know the geometry of the place cause it might take a lot of walking/trams). I show up usually around 65 minutes before departure at SFO (international flight with checked bags); never had a problem.
> you can totally show up 20 minutes before a flight
I'm assuming you're talking about the scheduled boarding time, not departure time? Most airlines' contracts of carriage say that if you're not at the gate 15 minutes before departure, you'll be denied boarding.
EDIT: I guess it's not clear whether you meant "show up [at the airport]" or "show up [at the gate]".
