How soon is too soon to find a new job?

It's all in the phrasing.  Your resume should read that you work for XYZ Corp. for nearly two years as both a (what you did) and a (what you do).  You would like to have an opportunity to discuss employment with ZYX Corp because you know of the company's reputation for quality and for way they treat their employees.  (Have examples ready, you'll be asked)  Focus only on positive things.  You like your current job but because of their reputation you would like a job with them better  

A year is sufficient.  In fact, I read an article a while back about the biggest career mistakes people make.  One of them was staying in a job too long.  The author's point was that every year when you get your performance review, you should privately be doing one of the company.  If they are not meeting your needs in some way and you don't have a clear path to fix that, then move on.  The company would have no problems dismissing you if you weren't meeting their needs.  I agree with this author's opinion.