The maximum period of admission into the U.S is three years when a TN enters at a US Border.  There is no limit on the number of years a TN visa holder can stay in the United States.  However, the TN visa status is not for permanent residence. 

Extension of Stay
For Canadian or Mexican citizens admitted as a NAFTA Professional may seek an extension of stay, which may be granted up to three years:

• Applicant may apply at a port of entry using the same application and documentation procedures above as required for the initial entry; or
• If the applicant is in the U.S., employer may file Form I-129 Petition for Non-immigrant Worker with USCIS