Well, i do agree playing for 6 to 8 hours with personal commitments such as study and work can be rather unbeliveable. I practice a juicy 6 to 7 hours a day, however I am preparing for conservatory audition, and I do not have a full time job ( I just quitted it to free up my time ) , basically the whole day i do nothing but practicing piano, so you can consider me a full time pianist.
In contrast, before I quitted my job and decide to practice full time, I could barely manage to sneak in 3 hours a day at best, most of times I would just be happy with 1 or 2 hours . I find that after entire day of study and work, you will be physically and mentally too drained to practice. Instead of using the precious little time that I had after work, I had to drag myself to practice piano, which ended up in a period of nervous breakdown for me.
Playing piano is physically and mentally very tiring. It takes really strong mental determination to practice very long hours. I practice in blocks of one and half hour, with rest of half to one hour in between, and I find that even supposedly much freer time i have, it is not easy to attain practice hours above 3 hours. In fact by the time you cross the 2 hour mark, your mind will be so tired that you just want to sleep ( I always sleep during breaks to refresh myself ). Being always fresh is important in practicing long hours, that's why I think only full time pianists can practice that long, because they have plenty of time to rest between sessions. Personally, I don't believe that you can practice long hours with many other commitments, I just don't see how anyone can take that amount of stress ; I know because last time I couldn't take the stress.
And one more thing, more practice hours do not always mean necessarily good. I am practicing long hours because I spend a lot of time on techniques and preparing my audition and performing repertoire. When my techniques are more secure, I will reduce my practice time to around 4 to 5 hours, there's no point practicing long if theres no reason to justify it.