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The rank of Instructor is a tenure-track title. It is used for probationary appointments only, for example as part of a contingency for a tenure-track hire made before completion of degree requirements. See COE: Degrees and Contingencies.
Per HOP 2-2010, Academic Titles and Tenure:
The maximum probationary period that may be served in the rank of Instructor is four years. Not later than an instructor's third year of full-time service, the budget council and chairman of his or her department shall recommend to the administration: (a) that he or she be promoted to assistant professor; or (b) that he or she be placed on terminal appointment for the next (i.e., fourth) year. The instructor should be informed not later than August 31 of his or her third year of full-time service of the final decision regarding his or her status. Nothing, however, shall preclude a review of a decision to place an instructor on terminal appointment during the year in which the individual is serving under terminal appointment.
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