Been taking a mens one a day vitamin now for a week. how long before i notice a difference?

Depends: The results you get from a vitamin depends upon many things. One, it would depend on your body's need for the vitamin - if you do not need it , it will not make a difference. It also depends upon the the quality of the vitamin- some are good and some are no good at all. Next, it would depend on your body's ability to absorb and use the vitamin. It can take months to notice a difference.