how long do you have to live on a piece of land before it becomes yours?

Adverse possession (squatter's rights) requires continuous, hostile, open, notorious, and actual use of the property for the statutory period. 

In TN, if you have color of title which means a document that appears to give you title to the property, but is not valid, the statutory period to establish adverse possession is seven years, but TN requires that the document be recorded for all seven years.  Recorded means filed with the County Recorder's office.

In TN, if you don't have color of title, in other words no document that purports to give you title to the property, then the statutory period is twenty years.

As mentioned above, adverse possession requires continous, hostile, open, notorious and actual use of the property for the statutory period.  Continuous means using the property for the entire statutory period.  You and others can do tacking which means you used the property for part of the statutory period and someone else used it for the remainder of the statutory period so that the total time equals the required seven years with color of title or twenty years without color of title.  Hostile means using the property in derogation of the true owner's rights.  Open and notorious means that the actual owner has notice of your use of the property.  Actual means actual use of the property for the statutory period.