From USCIS website.
"Q: For how long is a person granted TN classification admitted?
A: Previously, aliens admitted in TN status are admitted for a maximum of one year and by the end of that period must either seek readmission in TN nonimmigrant status or apply to USCIS for an extension
of stay. This final rule will extend that period to a maximum of three years to be consistent with other nonimmigrant worker categories, such as the H-1B."