Vitamins alone will not cure an anemia. The iron may start benefiting you immediately upon digestion of the vitamin. But what do you mean by "notice a difference"? A difference in... what??
I don't know what form the iron is in One-a-Day vitamins, but if it's not in an organic form such as "ferric citrate", you're wasting your money. (If the iron is listed as "iron oxide" or "ferric oxide", you're eating RUST!)