I think it's right around the same time a singer becomes a vocalist, a guitar player becomes a guitarist, and a drummer becomes a percussionist.
Is this a serious question?
Just an opinion! I'm using the terms as differentiation for seriousness/competence with the instrument. Lots of people here seem to not use the terms in that way. That's just how I interpret the terms! For those that DO see them as being different, I wanted to hear their criteria for a bassist, bass player, so on.