how to practice 6 hours a day? well, some people started building up 3 hrs for a week, ten 4 the next then 5 etc. i did that for a while, but i stayed at 4 and kept feeling tired. so i lauched myself into the deep end and started doing 6 hrs every alternate day, and 2 hrs singing on that free day. that way i was allowed recovery time. i did feel smashed after 6 hrs, but after a while you can just do that 6 hrs a day. however, the crucial thing was to draw up a timetable including all the breaks, and assign tasks for each hour, so you are actually doing 6 proper hours of practice, as opposed to 6 hours of time wasting. also, it makes the mind work better if you know exactly what you are doing, how and where.
you might not agree with the 6 hours alternate days, but i think everyone has their own way of getting things to work.
Gruff