Hi DianaLSmith and welcome to the forums,
Short answer - NO time limit - provded you remain a Comcast customer.
DId you by chance have a retail Norton product installed prior to installing Comcast's Norton? It could be picking up that previous license - we'd need more info before being able to assist further.
Feel free to come back with more questions/info pertaining to your situation if the "short answer" does not meet your needs.