Dear Genghis,
I can only speak for the DC-10 and the B767.
We would use airframe as well as engine anti-ice for all take-off's in icing conditions.
In the air, in icing conditions, if we found sign's of ice build up, ie on the wiper blades, we would use airframe anti-ice as well as engine anti-ice.
Boeing and MD, also had different definitions for icing conditions.
The % of the flight in icing conditions is difficult to define. You may fly from the UK in the winter and not need it at all. You could also fly from Singapore to Sydney, and due to the ITCZ, be IMC at FL 290 and due to the high temperature require engine anti-ice, as well as airframe.
Hope this helps.