You mentioned the operative word. Muscle. Like training any muscle you would not want to do the same excercises every day. One day work it hard and focus on strengh and endurance. Then the next day focus on speed and agility. Then the next day perhaps a light or soft practice...and so on and so forth... To build a muscle, rest and recovery is just as important as working it hard. As far as how long you should practice I guess that depends on the cumulative whole of how much playing you are doing. For example if I am performing a lot or involved in a taxing job (i.e. lead/commercial work) my practice sessions are fairly short and more or less just maintenance. If I am only doing light work or just performing weekend jobs my practice sessions tend to be a little longer and in depth.