I don't know about planes, but UFO's can travel almost infinitely. Their guidance systems can be disrupted by lightening, possibly by radar, and HAARP signals, though . . .
Learn to spell Airplane first
that depends on the aircraft, original fuel load, winds, altitude, all up weight. etc.<br />
a 747 400 series with full fuel and a load of 60% max has a range of about 15,000km. and that is at around 900kph.<br />
so around 16.5 hours maximum.
While it's not yet commercial, there is a plane that run on batteries that can fly all night on a charge, and has solar cells so that it recharges itself the next day even if there is cloud cover so theoretically infinately, barring mechanical breakdowns. With commercial aeroplanes, assuming in an flight refueling setup, the answer would be the same. Forever or until something breaks down.
My guess would be about a day max if the fuel tank was full at the start of the flight. Other factors such as weight of cargo/passengers and how much fuel the plane started with could affect how long the plane flies.
I dont know so much...Im only a student who is still learning...