Is success due to hard work sustainable in academic research? I am a physics master's student hoping to pursue physics research by doing PhD, post-doc, etc., following the standard path. To put it straight, how far can sheer hard work take one in academia until it becomes impractical and affects one's personal life?(in terms of research quality and quantity)

To anyone who is interested to know why I ask this, please read further:

Since high school, I have been working super hard to succeed and have managed to reach my goals and actually do things which I didn't think was possible(personally) a few years back. I am in one of the best places in my country to study physics, have (what I consider) good grades and some great research experience through which I  even got to travel to different country. However, this has been mainly due to my hard work and my professors/guides  looking at my hard work doing favors by giving great recommendations, calling up other professors for me to work with etc. I have almost consistently put in at-least 10hours per day(including weekends) since my high school and there is literally no way I would achieve any of this without putting in this much time. 

Until a few years back I thought this is the way but now I starting to reconsider. I am starting to believe this is impractical in the long run in research where I put in over 12hours of hard work with moderate intelligence. I have seen some of my classmates who are obscenely smart and the only way I can even come close to their level(in terms of grades) is by sitting in library the whole day. I do not know how far this hard work can take me and even if I it does is it practical. But if I stop putting this much work my grades will definitely go down and I would be letting down my professors. But to do well I feel like I have to put more effort the more I go forward.
I can't say whether 10-12 hours per day is right for you, but caution you to consider your health. If it suffers, then everything will suffer. 

But "intelligence" alone is overrated. The path to Intelligence, actually, runs through Hard Work. 

I once got the results of an IQ test (I hope they don't do this any more). The printed results said that I was a bit above average, but not outstanding. They recommended that I set my goals at community college rather than university as I'd be more likely to succeed with more limited goals. 

About fifteen years later I earned my PhD in mathematics. So much for prediction. 

More about me. When I was young I was pretty smart but was disengaged from school. My life was elsewhere. I saw no point whatever in school. It was in the second or third year of secondary school that I had what I considered my first positive educational experience. But then, I started to work. And I worked hard. 

But, you need to use the hard work to seek insight into what you are studying. The work, itself, isn't enough. But the insight is very unlikely to come to most people without the hard work. 

Again, though, don't neglect the wider picture. Get enough sleep. Get enough exercise. Seek feedback on what you do from those who know more. Find a mentor (or three). Ask a lot of questions. Take a lot of notes. 

And don't try to work past the point of frustration. The work will be very inefficient. Take a break for a bit. Then come back to it.