I'm in the middle of P6/Grade 5 exhibition with my students. They are intently focused I would say about 80 percent of the time, but sometimes their eyes roll back in their heads or I actually catch them passed out in front of their computers.
(In another blog I'll write about how important it is during exhibition for teachers and mentors and parents to help in the finding of sites related to their research so students can actually focus in on the research rather than spending the entire six to nine weeks on google fruitlessly culling through millions upon millions of useless sites...)
Meantime, when my students get exhibition fatigue, I give them permission to step out of the room and go for a run. They just need to look up at me, give me a nod, I give them a nod back, and they know they are allowed to scamper out onto the field for five minutes to clear their heads. I have some students do it two or three times a day; some students never do it, and most students do it a few times each week.
What I do know is this: each time a student comes back, they're ready to get right back into what they were doing with renewed vim and vigor. Either that or they've realized they need to be doing something else, and they get going on another aspect of their work.
Giving students permission to take responsibility for not only their own learning, but their own need-to-take-a-break time is important. My ten and eleven year old students can handle it. I trust them. They haven't let me down yet.