Restylane a filler within the class of hyaluronic acid fillers. Restylane's longevity is variable, but you can expect it to last from 6-12 months under your eyes. Juvederm is also a hyaluronic acid filler, and I find that it lasts longer than Restylane and works better around the eye area. As a treatment for the very dark circles under your eyes, any filler may be disappointing for you. The fillers work very well in that area along what is called the "tear trough", which is a hollow area that is formed along the rim of the eye socket. To the extent that the dark areas are caused by shadowing from the tear trough, the filler will help somewhat. However, these dark areas are generally caused by discolorations in the skin, which will not be helped by filler. You should be evaluated by a board-certiied plasic surgeon to see what would work best for you.