I think it varies with both the instructor's and student's individual experience levels. We all know how much easier flying becomes with practice, similarly with practice instructing does too. Likewise an 'experienced student' will require less effort than a real beginner.
Bearing in mind that instructing is the equivelant of single pilot, but more demanding, I would say the CAP 371 rules (for SP ops) minus a bit would seem a good guide. With briefing and debriefing thrown in, 6 hours of flying is a pretty busy day, I'd agree.
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Another day in paradise