There is no height at which ground effect is switched off. It just becomes less and less effective. There are two factors which determine how strong ground effect is.:
- Height relative to wingspan: The downwash cannot move into the ground so the induced flow field around the wing is distorted. In a first order approximation, the wing affects mostly the air which flows through a stream tube with the diameter of the wing span. For an explanation see this answer. Once the height is above one semispan, the distortion of the flowfield comes to an end.
- Trailing edge height relative to wing chord. For very small heights, the gap between trailing edge and ground becomes small enough to more or less block the proper exit of the flow below the wing. The result is an increase in ram pressure on the lower side and further distortion of the flow field such that the stagnation point moves down and the suction peak of the nose flow becomes stronger, adding nose thrust and reducing drag.
When moving up from zero height, ground effect diminishes rapidly because the gap between trailing edge and ground opens up and the ram effect vanishes. Moving up further will also weaken the first effect until at a height of one semispan the flow will be very similar to that of the airplane out of ground effect.
To illustrate how much happens on the last centimeters (or feet), look at the two pictures which I took from this YouTube video:
In the top picture the wing is clearly closer to the ground than its semispan, but the vortices from the flap edge trail straight behind the plane. Now move forward to the lower picture when the trailing edge is maybe half a chord above the ground. Now the trailing vortex is swept to the side because the flow, which is compressed below the wing, expands when it exits the gap between wing and ground. Additionally, the downwash becomes a sidewash, because it has no space below the wing left in which to drop.
This should demonstrate that most of the ground effect happens when the ram pressure below the wing builds up. Once the trailing edge is less than half the wing chord from the ground, this part of ground effect kicks in, and the aircraft sees a significant reduction in drag.