Articling Students

Rachel recently graduated from law school and got a job as an articling student with a law firm in a different city, i e, not my home town. During the hiring process, the firm made it clear their en goal is to hire articling students that they will keep on as associates. I planned to article with this firm, get called to the bar and then move back to my home city to work as a lawyer with another firm. What is the etiquette for revealing to your boss that you do not plan on working there permanently? Should i ease him into it by dropping subtle hints throughout the year? Or should i wait until my articling period is over and then decline the offer