I'm pretty sure that my longest project was Miss Hathaway's Garden from Pegasus Originals. It was 6 years from the time I started until it was finished. Here's a picture of it for reference:
I did do other things during that 6 year period: several afghans, small cross stitch, possibly some quilting. There were probably months at a time where I never touched MHG.
I'm sure that since I've discovered HAED, I'll surpass the 6 year mark on a project, especially since I currently have 5 of their patterns in my rotation!