. . ."there is the basic rule in almost every FOM that a lookout (also in cruise etc.) is mandatory.
Are you a cadet pilot, or is this something new? . . . as I have not read nor been told that a "lookout" was mandatory in cruise; nor have I ever heard of any crew taking turns to look out of the non-covered/non-shaded window(s) for 12 hours. . .
As a matter of fact, in cruise I'm usually kicked back; reclined in the chair with my feet up on the dash reading a magazine . . . .