It depends on the type of plane. Passenger jets typically cruise at FL 320 to FL 450 (32,000 to 45,000 feet). Piston powered planes will cruise at lower altitudes, which will depend on turbocharging, pressurization, etc.
For commercial jet airliners, cruise altitudes tend to be between 30,000 and 40,000 feet above sea level. Very short flights may fly at lower altitudes, simply because the fuel cost to climb to high altitude may be greater than the fuel savings of flying at high altitude. Two hours isn't really a short flight, though. In some cases, aircraft may fly lower if the prevailing winds at higher altitudes would mean flying against a brisk headwind for the duration of the flight. The slightly greater fuel cost of flying somewhat lower would be offset by the savings of not having to fly against the wind.
For small private aircraft (excluding jets, which are like airliners), cruising altitudes are usually well below 18,000 feet, and altitudes of a few thousand feet are not unusual. On very long endurance flights, higher altitudes may be used, such as 14,000 to 17,000 feet. A two-hour flight is not very long, however.