Eww. When God tells you to! I know some musicians that got tired of playing where they were playing and some that just wanted to get paid "like everyone else". Most of these people left too early, and God stunted their musical growth. They then wonder why some things are too hard for them to play or comprehend. Remember, if you're in God's will, no matter what you're doing, he'll make a way for you. If you're a musician, and your choir wants to sing a song, he'll show it to you, whether he has to use someone to show it to you, or even if he shows it to your through your practice sessions. I can use myself as a testimony for this.
When I first started playing in church, I was taught by the resident minister of music. After three years, I ended up being the main musician, though I never received the title of "minister of music", and this was ok with me. I then started to receive a lot of offers to play with other choirs and at other churches. This same thing is happening to me right now, at the church that I play at, there is already a competent musician, a korean one at that, and he's pretty good too; however, it's working out that I'm leading more and more songs. I thank God for this, because I love to play, and I love it even more when I can feel God guiding my hands on the instrument that I'm playing. All that said, let God lead you and tell you when and if to move. You'll get better results that way.
(We have a pretty big bad, in my opinion, 3 keyboards, my organ, a drummer, a bass, and a sax player.)