To answer the OP, you remain within ground effect up to about 12 knots, where translational lift takes over. With a fixed wing aircraft, ground effect only occurs because the wingtip vortices are reduced. On a helicopter, you also reduce the induced flow coming in from above the disk (it bounces back from the ground), so the angle of attack increases for two reasons (this is propeller theory, only sideways).
And to anticipate another question, translational lift occurs because the air is flowing into the disc by itself, so you don't need to use so much power to suck it in, which also reduces the induced flow (and increases the angle of attack of the blades).
To answer another poster:
"I've always been under the impression that if you cannot hover you shouldn't takeoff."
Indeed, that is the EASA rule.