Once legally free how long does it take?

My wife and I are in the process of adopting and are in the transition process. The child (19 months) is in the foster care system. Right now the child gets to stay at our home 4 nights and the foster parents 3 nights. This has been going on since July first. My question is how long should this take before the child is placed permanently in our home? All paperwork on our part is taken care of. The child seems to be transitioning okay. He has become clingier at both homes but no other serious red flags.