Yes, 12 months is the limit, at least in State of Florida.
I had an American friend who used to live in Ohio, when He moved to Florida, He had to wait almost a year before the State acknowledge him as a Florida Resident and can go to school in Florida as a Resident and not out of State Resident.
I don't think immigrants will get special treatment. Once you stay in one State you need to stay at least 12 months before considered a resident of the State.
I know there is an exception but I only seen this for Americans (not immigrants). If you're not a resident of the State but your parents are, you can be considered as a Resident of the State based on where your parents live. This mainly has to do with where the income tax is generated. You will only get benefits from that particular State like in State Tuition if you or your parents pay taxes to that State.