How long does a foster child stay with you?

What is the longest any of you had? I also have another question. As I said I want to foster to adopt. I know that I have two options there. I could only accept children that are already legal free or I can accept children that are a legal risk. IF I do take in a child that is legal risk. What exactly does this mean? Does this mean that the courts are already in the process of terminating the parental rights or it's probable that the rights will be terminated? I know there is always a risk but what are the chances that the child would be removed and sent back to bio parent or family members in this situation. Does it happen alot? My other question is I want to adopt girls but two dif age groups..We would love to have a daughter in the newborn to 4 age group as well as adopt a girl that is in the 12 to 16 age group. Is this possible? The reason for this is we have two teenage daughters that are 16 & 15 and would love to have a daughter that is close in age to them..we also have a son that just turned 5 this month so would like to stay in his age group. My husband and I only have one child between us which is our youngest son that we adopted.. My other question. Is we want to foster to adopt 100% possitive on that..what we are still discussing is fostering..So how would that would if we did both?