How long does a foster child stay with you?

I have had them from 12 days to 23 months. The 23 months situation-it started out that they thought they would be returned within 5 months-it ended they never returned but are being adopted by maternal aunt of their's. All of the children I have had have been from my county except two cases. 1) They didn't have any available homes to keep 3 little siblings together and 2) The kids had to be moved out of county due to HIGH violence in almost entire family and threat of fleeing with them. They usually try to keep them in the same county to make it easier for visitation purposes even if the parents aren't getting any visits right then.