The longest achievable groundwave range is about 40,075 km, or in other words, the circumference of the Earth. There is no longer possible path over the ground.
In other words, there is no upper limit to how far radio waves can propagate, though there is a limit on distance imposed by the geometry of the earth. This holds for any mode of propagation, including ground wave. It also holds for any frequency, including the 40m band.
All electromagnetic radiation becomes progressively weaker with distance according to the inverse square law. At some point it becomes too weak to be detected, but this can always be compensated by using a more sensitive detection method, or increasing the transmitter power. As such, there is no limit without defining additional constraints.