This varies from casino to casino, but in general, you have to gamble large amounts, with consistency, to be classified as High Roller from a casino. For example, in slot games High Rollers tend to gamble at least 25 euros per spin, and for card tables, they tend to gamble 100 euros per hand. This also means that if you are considered in a casino as a High Roller, this only stands for this particular casino that classified you as one.