Below is the answer for your question:
How Long Can I Stay?
The maximum period of admission into the U.S is one year. The US Citizenship and Immigration Services (USCIS) grants extensions of stay in time amounts of one year. 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.
This is from the URL: http://travel.state.gov/visa/temp/types/types_1274.html#21. It is a little bit out of date since the maximum period has been changed to 3 years.
How Long Can I Stay?
The maximum period of admission into the U.S is one year. The US Citizenship and Immigration Services (USCIS) grants extensions of stay in time amounts of one year. 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.
This is from the URL: http://travel.state.gov/visa/temp/types/types_1274.html#21. It is a little bit out of date since the maximum period has been changed to 3 years.