i tried practising for extremely long time, about 6+ hours, it's very tiring indeed, and the yield is not much more than 4 hours of practice.
i would like to share my view about practising long.
firstly , i think it wouldn't be possible for an ordinary human being to keep being concentrated for so long, personally i would develop headaches if i went straight for 4 hours or more, which effectively prevents any further progress.
i think a way to work around this is to plan my practices, i experimented with a few "ways" to practice. One very important thing i noticed is that, including a break in the right time would be a very good idea.
Where to put the break in my practices? I do some brainstorming and thinks that it won't be wise to practice straight for more than 2 hours because every following minute is worthing less and less. So why not have a break after practising about 2 hours? a break of 45 minutes+ would resume my mental status and makes me more "motivated" to practice due to "boredom" in my breaks (actually not boring... but i can't describe it in words)
To fully utilize the break times i sometimes treat my lunch time as a break, that means practising for a while in the morning could be a good idea to use my time.
Assume i started practising at 10:00 a.m. , having lunch at noon, i practiced for 2 hours, i resumed practising at 1:00 p.m. till about 3:00 p.m. , i take another break of an hour, then practice from 4:00 p.m. till 6:00 p.m. Then i have practiced 6 hours yet possibly yield much more than practising straight for 6 hours.
of course, if you're allowed, using the dinner time as break would be even better, that means you got a rather free afternoon for things other than piano, unfortunately, my parents don't allow me to practice in the evening/at night.
Other than breaks, i somehow thinks that a well-planned practice is also a way to reduce tireness and keep myself focused, i would not practice difficult things (to me) for every hours, i would make it more balanced with some more easier and relaxing things, if i couldn't practice a difficult thing for , say, 3 hours, why not cut it into 2 hours and enjoy my extra 1 hour playing easier stuffs?
so, when i plan my practices
Morning Practice, about 2 hours: i make use of my concentration and work on most important things, i have only 1 morning practice coz people would kill my if i practice too early

Break: Lunchtime + free time.
Practice(1) about 2 hours: things that don't demand much concentration
Break:free time, perhaps some websurfing
Or one large break and go somewhere else for fun.
Practice(2), about 2 hours: things like technical drills, or if i take a break long enough i could practice things more important again.
then it's about dinner time, after dinner i can't practice any more so .. well, 6 hours is my limit.
I somehow thinks that 2,2,2 practice isn't the best but i have no choice. 2,,1.25,1.25,,,,1.5 would even be better. ("," means breaks), 2 hours in the morning where i have highest concentration, then two 75 minutes practice somewhere in the afternoon where i am free enough to find just 2.5 hours for practice, and one 90 minutes in the evening, when i regained energy and concentration after a tasty dinner

**dreams**
I don't speak English well... i hope you understand my point.