Can't comment on fixed wing but in my experience accepting Eurocopter products I was presented with accepting aircraft which had been fully post production tested by their own production test pilots. These guys had done an abridged TP course at EPENR and flew with FTEs over about 12 hours or so (would vary with type I guess).
Customer acceptance is generally supposed to be a formaility and done by the company's Chief Type Captain alongside one of the PP TPs or Instructor as the a/c commander (still restricted F reg). Any snags are then corrected before final acceptance. In some cases (normally private owners who don't feel qualified to assess) the acceptance is purely done by the manufacturer.