Being in a sector myself that involves a fair amount of sight-reading and quick-learning (I'm working on a graduate degree in Collaborative Piano), I can tell you that one of the paradoxes of sight-reading is that, beyond losing the fear of playing something unknown, it has mostly to do with how much music you have learned (and learned well), and less to do with time spent sight-reading. A large part of sight reading has to do with recognizing patterns, and so the more that you have in your ear/fingers, the easier it will be to play them at sight. For that reason, we are continually improving in our reading abilities . . . On the other hand, it is a good idea to put yourself in these sorts of positions: playing for auditions, accompanying singers, reading chamber music with partners, etc.
Hope that helps,
Mike