David Sylvester - From the Models category:
-on posing for Alberto Giacometti...
The sittings would begin between 3 and 4 o'clock and go on till about 7:30, when the light went... There were 20 of those 3- to 4-hour sittings for my portrait... His glance moved very quickly, almost continuously, between canvas and model. (David Sylvester)