if i were to start to self teach myself how to play piano now, at age 15, and practise 1hour a day everyday, how long will it take me to learn piano fluently?
It depends on the person, but no matter what, it’ll take you a long time unless you are highly gifted.
However, you will learn a lot faster if you have a good teacher. i recommend getting a piano teacher, a highly regarded one. If you have a bad teacher, or a mediocre teacher, you won’t learn as quickly. If you are determined to be self taught it will take you a lot longer.
I started piano at 14. I took lessons from an amazing teacher and I’m "fluent" (like the term used that way!) enough in music to major in piano in college (I’m 18 now) However, I’ve been reading music since the fourth grade, which helped. When I first started piano, I only practiced for a half hour five days a week, now, I’m obviously years behind (most piano majors have been playing since elementary school), so I have to work hard and play an hour and a half every day. Eventually i’ll try to get it to the point where I can practice two hours, maybe even three hours a day without hurting my hands.
Just remember, though, there are different levels of playing and different levels of reading and technique, too. Just develop what you want, be it technique, reading ability, or musicianship- there are many areas in which to judge an instrumentalist. Completely mastering piano takes a lifetime, because you can always learn from it. Best wishes and good luck to you.