The normal dose for evening primrose oil is 1000mg to 3000mg/day. Like the article said, the dose should be broken up into two or three doses. So 500mg twice a day or three times a day.
If the supplement is taken in liquid form, then obviously the doses are different. The liquid doses are usually much smaller but it can vary from product to product. The label should mention how much of the liquid is equivalent to how many milligrams of primrose oil.