As discussed in the previous question, what length of time is considered reasonable will generally be analyzed in conjunction with the other factors. For example, if the non-compete agreement is designed to protect valuable information the reasonable duration is for the time the information has value.
Lifetime bans on a particular area of work have been upheld in unique circumstances, but, generally, courts analyze the protectable interest the employer has and will not uphold time restrictions which go far beyond the employer's protectable interest. For more information about the employer's protectable interest, see the next question.