Where did your relationship with music start?
How long have you been making music?
How long have you been on the music scene now?
How long have you been performing?
How long have you been on the music segment side? Where did it all begin?
So, Not-Robot, how long have you been singing?
Now with since:
Since when have you been a singer/are you into singing?
but be aware that with by using SINCE they make questions used to require an explanation, or it might indicate surprise etc, as in Portuguese we say "desde quando você é cantor(a)?" [tipo "não estou acreditando que, você com essa voz? não é possível ! e tal...
To answer you can use SINCE freely, but it´s not advisable to try it with questions.
So, "since when have art been in your life?" is perfectly grammatical, but the meaning is different.