The Adoption and Safe Families Act of 1997 specifies the goal is to seek a permanent home and family for the child as quickly as possible, whether achieved through reunification with the biological parents, kinship care, guardianship or adoption.
Timeframes will vary.
A child may stay in a foster home until his/her/their biological parents are able to care for them again, until a suitable placement can be made with a relative or until the child can be adopted. On average, this tends to take 12-18 months, but each case is unique.