How many hours per day do I need to work to become a very good PhD student?
Uri Alon has a lot to say on this subject. See from 6:00 onwards in this video, the whole of this video, and the first 2 or 3 minutes of this video (they are all from one talk broken into separate videos). 

The way he once described it to me is this: According to the parento principle, you get 80% of the benefit for the the first 20% of the effort. But productivity doesn't just keep increasing if you work more.  If you do 0 work, you will fail. If you work 100% of the time you will also fail, because you will die. Thus there must be a curve with a peak in between 0% and 100%. Because its a peak there will be two x values (time spent) that give each y value (productivity): working slightly less that the optimal time will give you the same amount of productivity as being slightly to the right. 

Thus it is always better to be slightly to the left of the peak as you get the same productivity with less effort. Where that peak is will be different for different people, and you will have to find your own peak, but many people stop working harder when they realize that putting more effort in is making things worst rather than better, and thus are on the right of the curve. 

The other reason for being on the left is that if you are on the left of the curve, you wll have to reserves to give a little extra for a short term boost when its needed. 

I don't know where your peak is, but for many people its probably at 40 hours or less a week. Remember this is actually time doing your work, not just being present in the office.  Just make sure you are working for those hours you think you are working, and you are taking proper time off in the hours you are not.