I'm 24 and very interested in learning piano (to a professional level). I've never been to any class and I have to start from the scratch. Is it possible to learn piano on your own? How long does it take to complete grade 6 (Trinity or ABRSM or any)? I can practice 2 to 3 hrs a day.
[assuming you meant "learned"]
The simple answer is yes --you can learn to play piano without formal lessons. The folks on Q&A sites like this one can help you, as well as the abundance of online resources (and books!) devoted to piano instruction. (Patient friends can be helpful too.) The good thing about self-teaching piano is that it's easier to learn by watching than, say, a wind instrument, where the minute details of tongue placement make all the difference. YouTube is also full of pianists trying to share their knowledge. Services such as Google Helpouts have some on-call people offering musical instruction (typically not free).
Would it be easier to learn with a skilled teacher? Yes, but in some ways, it is more of an adventure to just learn to play what you want to play --as long as you are passionate about improving and learning more, you should progress. Since you seem prepared to devote some serious time towards mastering the piano, I think you will eventually become a fairly competent pianist in time, if you put a lot of heart into it. Internationally famous? Nobody could promise you that, even if you were taking lessons from a world-class instructor.
If you're going to self-teach, just remember that there's typically a lot of research involved (mostly a lot of Googling). (But that's part of the fun!)
Well, my 2c is that if you can possibly afford a teacher, you should learn with a teacher.
Ask around on reddit and pianoworld and the vast majority of the self-teachers (including me!) will tell you they wasted a lot of time on their own.
Piano is a very deep art. Your life is only so long. You should get a teacher if you possibly can.
Can you learn on your own? Sure. I did. Learned a lot.
Should you? No. I wasted a lot of time getting traction on what a teacher would breeze me through in a much shorter time.
Spend your time learning songs.
Don't spend your time just coming to grips with standard notation and chord progressions and common styles and fingering and performance stuff (but wait, there's more...)