I believe that the quality of your training and how you use your available time is the most important thing, I manage about 8-12 hrs a week in three arts and although I know i would benefit from more hrs its just not always possible so i try to focus on what i really want form my training and train specific skills each session. I tend to think about Martial Arts all the time anyway and as long as you have that active thought process and desire to learn you can still achieve good things even if you can't do 8 hrs a day.
ps: I Just noticed my rank at the top says white belt which was true in 1999, anyone know how i can change ti as I can't work out how to edit that bit?