well, It all depends on how good you want to be and how fast you want to be that good.
I say, play as much as you can each day, without distracting from other duties and passions, and never any longer than it remains fun...I tried to push myself to play hours and hours daily, but after about four hours a day, I stopped enjoying it, since then I slowed down, and now play for as long as possible each day, which is usually close on an hour or so.