I noticed no time limits, i.e. 1 year from start date. My definition has been very loose: January 1 and lanquishing.
I like Rob's: "I count it as a UFO if I’ve actually cut the fabric, however, most of my UFOs have varying degrees of sewing completed. I love quilting, but am not as fond of piecing, so my UFOs are mostly partially pieced tops rather than flimsys waiting to be quilted. I find that once the piecing is done I am motivated to go ahead and get it sandwiched and quilted.
As far as timeline goes, most of my UFOs have been sitting unworked on for at least a year, including a couple that are nearing the decade mark and one that is (ahem) 30+ years old. But I have had projects that became UFOs in my mind after just a few months of not being worked on. Unworked on and not thought about are the keys for me deciding something’s a UFO."