It depends on what level of quilt show you are talking about. Quilts that make the rounds of the national/international juried shows are most likely never turned into bed quilts, even after retirement.
Most shows have limited windows in which a quilt can be shown...completed in the last three years, for example...so, after that, if they haven't been purchased, they are retired. Often the big winners receive purchase prizes, meaning they give up ownership of the quilt in exchange for the prize money. Other post-campaign fates were described by DogHouseMom above.