How long does a foster child stay with you?

O.K. I have a case scenario here..Say I am foster to adopt I have a young girl and an older girl placed in my own they aren't siblings or related at all. I want to adopt them both. Can I do this at the same time or would I have to wait til one is finalized before I start the process? Another question i have is..as you know i'm starting the process now so I will have a homestudy done. Now if I choose to adopt a child will I have to complete a new study with each child or for both or will the homestudy I did to become a foster mom will that suffice?