I practise from 11 am to 3pm most days, and from 11 am to 6 pm on occasion. Broken up by students arriving for lessons, and daily admin tasks.
Each session is broken up into 30 minutes. In-between I do something to refresh concentration: walk around the room, talk my my wife, get a drink or a small snack, teach a lesson. Then I am back to it. This is important, because though I can sit and play for seven hours in one go, it is not healthy or productive:
1) You increase you risk of blood clots sitting for prolonged periods daily. Not to mention the problems with blind repetition of passages (even if rehearsed effectively), can cause muscle fatigue or RSI.
2) When we practise effectively, we do not make errors; we play it correctly. Errors and miss-readings are a sign of mental fatigue or concentration loss.
3) The more efficient we are with the initial reading, the less repetition is needed.
There is a difference between playing for hours on end, and actual intense practise. If you are sitting playing through pieces for seven hours, you are wasting your time. If you are sitting thinking about, each note, each body movement, and the sound your making, this will be productive.
It is a myth that rehearsing at length (upwards of 1 hour continuously) will yield results; its not the time, its what you do with it.
In regards to your laborious etudes, if you take time enough to understand the music you are playing in your rehearsal, you will find you will not need to practise it so much to 'get' it, but your rehearsals will be for confidence and permanence. You will cut your time down from 8 hour slogs to a 1 hour reading and recital.