Judges in the court were ruling on a the case of a woman who booked to fly from Bremen in Germany to Asuncion in Paraguay.
Her initial flight with Air France (AIRF.PA) to Paris took off with a delay of 2-1/2 hours and she missed her connection to Sao Paulo, also operated by Air France. She arrived in Asuncion 11 hours late.
Air France was obliged to pay her 600 euros in compensation, but appealed to a German court. It asked the ECJ if payment should be made in the case of a flight delay of under three hours, but final arrival late by three hours or more.
The legal challenges relate to the EU's Air Passenger Compensation Regulation that came into effect in 2005 and sets out passengers' rights in relation to denial of boarding, cancellations and delays.
It covers EU airlines flying into or out of the European Union as well as flights departing from the region by non-EU airlines.
The European Court of Justice has previously told airlines to compensate travellers bumped off flights because of strikes, saying that was not a good enough excuse not to reimburse.
It has also ordered payments of between 250 and 600 euros for delays, depending on the length of the flight, except if they are caused by extraordinary circumstances beyond the air carrier's control.