Hi Shingo,
in my holidays i sometimes manage to play more than 4h a day. I usually start as early as possible which is 9 a.m. at the best. I found the early hours are the most productive, so i start with some technical excercises (Brahms) for like 15-20 minutes. After that i work on the most demanding sections of the pieces i am currently working on (for example the octave runs in Chop. op.23). After 2 hours usually my concentration is at a temporary low and i try to find something to eat. I'll pause until 3 p.m. and then start again this time focusing on musical issues (the truth is, you cannot really separate them from technical issues, but you know what i mean). I will hopefully play until early evening and at the end of the session i try to just play a few pieces of my repertoire, because its fun und this way you keep things polished.
A suggestion: If possible try to take a recording of the pieces you are working on. I find this is a good training for concentration. Because i am an amateur i don't get to play in public very often, so this way you can somewhat simulate a recital situation, having to play flawlessly AND with musical expression.