. . . [W]e find there is a positive pay-off to a firm’s decision to sign an endorser, and that endorsements are associated with increasing sales in an absolute sense and relative to competing brands. Furthermore, sales and stock returns jump noticeably with each major achievement by the athlete. However, whereas stock-return effects are relatively constant, sales effects exhibit decreasing returns over time. We outline implications for practitioners.
That is from a paper by Elberse and Verleun (July 2011), "The Economic Value of Celebrity Endorsements."