ShiroKuro I've tried it all when I was trying to push my limit haha. I managed 7 hours of pretty focused practice one day and got a lot done. It was 4 hours, sleep for an hour and chill, 3 hours in the evening. I hoped to keep it up but crashed the next day and the day after and couldn't focus much. Maybe it's sustainable for some.
With 5-6 hours, I tried to take mini naps every hour or two, I would wait to reach the point where I would slow down, rest for a while and get back at it. This went on for maybe 2-3 weeks. Walks help too but naps are better imo. Actually, I also tried running after practice to consolidate it better, which seemed to work, but was too lazy to keep it up.
The people I've known who spend 7ish hours consistently (some music students) seem to spend maybe 3-4 of those hours maintaining repertoire. They do have higher accuracy because pieces are more fresh in their fingers but it's not really high quality practice at that point.
Nowadays, I've resigned myself to getting in maybe 4 or so hours more consistently. Once I know exactly what to do (when I have a eureka moment), I occasionally spend a day where I'm thinking about it pretty much all day, but my practice hours at the instrument seldom go beyond around 5 hours.
I could be doing more, I think. What's frustrating and rather disheartening though is how much of that effort ends up "wasted" because you practice slightly better every month and have to change fingerings and technique often. Maybe it can work if the piece is solved initially.