Bava Batra 29 - The Length of Residence for Presumptive Ownership
Presumptive ownership of houses, ditches, and bathhouses - or any other property that yields benefits continually - is established after three years of uncontested occupancy from day to day. For other properties it depends on their normal use, for example, a presumptive ownership for a field is established after harvesting three crops.
Why three years? For three years after purchasing a property, a person is careful with his deed, but longer than that he is not careful to keep his deed unless his occupancy of the property has been challenged. Therefore, after three years of the unchallenged use of a property, we do not insist that the occupant produce documentary proof of his ownership; rather, we rely on his very occupancy as proof.