I started a 3-to-6-month gig in February of 2011. I finished it yesterday. Just a couple months shy of three years later.
As I started nearing the end of my (nearly) third year, I noted to my company that I'd like to move on and do other things. Don't get me wrong, I like my client, and the work was pretty easy, but it was just getting to be a long time doing the same thing. One of the reasons I became a consultant/contractor is for the variety of opportunities.
The client had some interesting things going on, and I had worked hard and gained the roles to be involved in those interesting things, but it's still a long time. I had considered my participation in those things, but in the end decided that another year (probably) would be too much.
The parting was bittersweet. I'll miss the gang and things to do, but I look forward to different opportunities and challenges.