I have an oil change for Friday, 13 months since my last oil change. I just turned 73,000 miles, c. 7K miles short of 9,000 miles since my last oil change. This is with Mobil full synthetic. I checked the color and it is dark, of course, but still very clear looking and smooth not going gummy. I think that full synthetic is good for 12K miles or more unless you really beat the crap out of your engine or spend most of your time in stop and go traffic. If you are on highways and the freeway a lot - I am - then every 6K miles or 12 months is a generous amount of caution.
But I won't deliberately go over 6,000 miles or 12 months. This time I was focused on 12 months between changes and the miles got away from me, a bit. I was actually surprised just now when I checked the miles in my last oil change last October and saw that I was that much over 6K miles. I went over the dealer-set 4K miles on an extensive road trip once, by about the same amount that I am over this time, and they didn't say anything. That was before KIA started putting full synthetic in too. I noticed back then that the oil got really dark, darker than it is right now, even between 4,000 mile intervals. Full synthetic is definitely a noticeable improvement on oil longevity.