my flight to Helsinki departs at 7:10 PM. How early my I check my luggage on the flight?

You can check in your luggage starting from the time they open up the booths. You may want to be there at least two and a half hours before your flight

Generally for International flights you want to check it in as early as possible, particularly if you have anything in your bags that might require TSA to hand inspect. At least a couple or three hours before is a good rule of thumb.

I would suggest getting to the airport within 1 to 2 hours before your flight and check your bag soon as you arrive to the airport.