In 'AUTO' the signs illuminate whenever the flaps are extended. Hence, after takeoff, the belts go off when the flaps are fully retracted. Similarly, on approach, the seatbelt signs would go on when the flaps begin to extend. However, most carriers will manually illuminate the signs on passing through an altitude in the descent; often 10,000'. Many operators will have the signs 'ON' for takeoff and manually switch them off when safely airborne, this keeps control of the cabin since if there were to be an engine failure/fire and a rapid return to airport of departure, the last thing we want is for the seatbelt signs to go off as we retract the flaps.
In turbulence, the signs will be turned on manually. This is subjective and in light-chop this will be done in consultation with the cabin crew who can report on what services are being carried out in the cabin and the levels of turbulence in the cabin.