For how long did Black Beauty have liberty?

After being sold from farm to farm and between various owners for the majority of his life, Black Beauty finally gets to spend his last days in freedom and at rest. After being terribly abused and driven nearly to death, he recuperates for a time and is sold at a fair.
At the fair, Mr. Thoroughgood and his grandson Willie purchase him. They treat him kindly and help him to recuperate further. If you consider this the beginning of Black Beauty's liberty, he remains with those owners for around nine months, being purchased at a fall fair and living with them until the summer.
Mr. Thoroughgood then sells Black Beauty to another family, which turns out to be the family of the now-grown boy who previously cared for him. They care for him and let him live a peaceful, free life. At the end of the novel, he has spent a year there.