Is In-Seat Rest used regularly where it is legal, and is it restricted by airline SOPs?
Too complex for me to give a specific answer, and this answer is assuming only airline transport not private flights or business charters.
Duty time limits are different for every country and may involve several pages of tables and definitions, airlines hire people just to track and comply with this. 


Time of day relative to the pilots home timezone. If they are working when they are accustomed to sleeping then allowable hours are reduced.  
The number of pilots on duty and length of single flight. A flight with 4 pilots is allowed longer shifts than 3 pilots, and a single pilot aircraft will restrict flight time per week more than 2 pilot aircraft.  
Total hours worked per week.
Time between rest breaks, length of rest breaks, and location of rest breaks (eg at duty station, break area but expecting quick response(awake), break area without quick response(sleep in bunk)
Shift length, generally the whole period between breaks of 8 or more hours on the ground.(8 hours will vary with jurisdiction, ie some are 7 hours or 10 hours) For example if starting work after holiday: work 8 hours, off for 6 hours, work 7 hours, off for 10 hours. In many jurisdictions this would legally be considered a single shift of 21 hours with a 6 hour meal break.
Days between full 24hour rest days or number of 24hour rest days per month.