Hi,
There are no absolute answer on how many year to become a great singer. Actually, it's depend on you.
However, The only way is to practice, practice & keep practicing. The harder and discipline you are on your singing training, the sooner you will become a great singer.
You have to train your voice smart and work-hard... Remember, work-hard does enough for now... Be "Smart as well....Try to find some guidance..
And you have to make sure you are practicing with the correct method or exercises. Your vocal chord will be destroyed and hurt if you are singing in the wrong method.
Go find a good singing lesson. You vocal quality will be decreased if you keep singing in the wrong method.
Here is the singing lesson that suit for the self learning student http://www.singinglikepro.com/singorama.html it's already in Version 2, i think it's better than the version 1 which I got it last year.
Good luck