I am going to be traveling and doing a stopover in Iceland, but I am not planning on staying at a hotel. How early can I check in if my flight is at 7am?
Checkin times are set by the airline, not the airport.
Most airlines have a 24-hour policy for checkin, but some have different periods. Check your airline's website to find out their checkin period.
The website of the airport will tell you: it's 2.5 to 3 hours before the flight, depending on the airline. This is of course for an at the counter check in. Online check ins are often at least 24 hours before the flight. Many airlines also allow check ins at a kiosk, and they're open 24h/day.