I'm not "Progressive," so I'm not sure how meaningful the idea of "White Privilege" is to me, but I think that a more thoughtful progressive person would probably say that it started some time in the 17th or 18th century, the era of European industrialism and colonialism and imperialism. They might also say that it started in the early 20th century when American immigration laws locked in the concept of a shared pan-European white identity. As for how to define it, I'm not sure. A liberal definition would be "Benefits Given to people of European heritage under law," but a progressive definition might go something like "The sociological and economic benefits of a shared recognized racial category with those in hegemonic power."