Some single guys with implants do not tell their partners at all about their implants, and most of the time they can get away without their partner knowing. It looks, feels and performs close enough to normal that they can finesse away the non-natural aspects. However, I'd intend to tell a provider rather than try finesse. With more than a casual feel, the pump, which is like a hidden third testicle, would be apparent. Also, while its nice to be discreet about pumping up the cylinders, it seems unnecessary and possibly unsuccessful in any case, to try to disguise that completely.
But, I'm thinking the best time is once I have met her, and I'm thinking that if she's fully informed about what it is and how it operates, it really shouldn't matter to her. My concern about informing her before we meet is a) how to tell her without violating any taboos about discussing or implying specific activities, and b) the possibility that she might not understand what its about and be scared off without a somewhat more careful explanation.
Of course, I have some concerns about how to discuss it in person too, without violating any taboos about specific or explicit discussion, though I think I know how to handle that. Plus, I'm currently scheduled for a 4 hour dinner date with someone who appears experienced, intelligent, and well reviewed, and I think the discussion will be easier when we've had a chance to get to know each other a little.