how much time is fair?

So here is my back story. Was supposed to be moving to TX upon graduation. Had a position lined up in the ER (my 1st choice area and where my heart is). At the last minute SO's transfer was canceled. So we stayed here. As it was June when this all went down there weren't a whole lot of new grad positions left. I found a position in a Tele unit at my local hospital, on nights. I liked cardiac during school and while I hadn't considered it before, it was between Tele and med/surg and I really didn't want med/surg. I do feel that since I'm starting off there is absolutely plenty to learn no matter what unit I'm on, and it will be a very beneficial experience. When interviewed the director asked me my first choice unit, and I told her ER. She asked me if I had any plans to transfer down in the next two years and I told her I hadn't really thought about it at this point. Which was true. We discussed it a bit, and she basically said that she'd like me to stay at least a year, but with the 2 year comment I'm figuring really she wants a minimum of 2 years. I do think that I ought to really give it a year to get the most out of it, and also to give them time since they will be putting the effort in to train me (12 week orientation). Hospital policy is that you can transfer after 6 months. So I'm wondering what an appropriate/fair amount of time is to give a unit before transferring (when transferring not because its not a good fit, but because of wanting to try something else)? I'm not making 'definitive' plans, as I may end up loving Tele and wanting to stay. Just curious what the standard is, and what the thoughts are on it. I want to be fair to myself, but also to the unit and director. If they take the time and effort to train me its not really fair to jump ship too soon.

Thanks!