in an underpowered 2.25 series 2A landrover....
less than 1% in the Sahara (Libya, Algeria, Niger, Maroc). 4X4 is engaged in sandy sections when the vehicle is clearly slowing down. At other times its actually better in 2x4.
As for the rest of Africa, on dirt roads in the rainy season in Kenya, Tanzania, Zambia, Mozambique - more like 30% - often for directional control.
On rocky stretches of bad, bad, piste such as NW Namibia (Kaokoland, Maroc), low range is useful even if 4x4 isn't needed for the traction.