One hour before does work often. But with checked luggage and the possibility of the US Government kicking back in ways to the airline for check-ins on the way back to the US even before boarding on a CPH-ARN-EWR routing, 60 minutes can be cutting it too close if there is a hiccup at check-in.

If online check-in works the night before to produce a boarding pass with seat assignments, then 60 minutes is plenty for this itinerary when either having no luggage to check-in or having standard checked-in luggage within the included free checked-in luggage allowance.