Theoretically Forever.....But.....!
That would certainly depend on the receiver to be able to prove. Signal strength deteriorates rapidly at the very least via the inverse cube rule. Longer wavelengths such as AM radio tend to follow the curvature of the earth, while Shorter waves say FM band travel in a straight line.
Longer wavelengths bouncing back to earth produce "skip" conditions, where you can hear a station well beyond it's normal range.