First the legalities of it. Restaurants that contract with oil collection companies are paying for the company to provide a waste container and paying for the company to dispose of the oil. The Restaurant doesn't care how or where. So if a person just drives out back can takes it out of the collection container, it is theft from the company providing the container.
I have never seen a oil collection contract so I don't know if there is any exclusive rights to the oil that prevents the restaurant from giving it to you the Bio-Diesel maker.
Restaurants like to save money as much as the next person and by giving it to you rather than paying someone else, they save money. When I met with the manager of the place I want to use, we just agreed that I could collect it prior to it hitting the recycling companies container.
As far as amounts of oil, I don't know. Each restaurant is different. The owner/manager would best answer that based on consumption.