for me i guess it varies- sometimes it is when i see the end in site- i put it aside and move on-sometimes right from the beginning- i will (think) this will go together quickly- i can get it done tomorrow- then tomorrow winds up being months later.
sometimes i hit a road block (need to order something- or run out of fabric and need to re-think it) and i set it aside--then a dozen projects later come across it again
sometimes i decide i really don't like it- or it's not coming together like i thought it would- or i simply hate making a primary block for it- then it can take me months (or years) to finish it.
sometimes its frustration- sometimes it's bordom, sometimes it's disorganization (like my 'Over the River' christmas quilt---i finished the embroideries 4 years ago---but misplaced the pattern- so the embroideries went into a box---then around this past Thanksgiving i came across the box---still couldn't find the pattern (which i found yesturday) but decided i wanted to get the quilt together-so i made it into a quilt- with my own ideas- blocks-plan---of course i find the pattern a week after i give the quilt away

my oldest UFO is one that was a 'class gone wrong'---and it overwhelms and frustrates me everytime i think about it---it is a large box with over $500 worth of fabrics in it---signed up for this class- spent a fortune on 27 yards of fabulous fabrics- silks, orientals, batiks- had to spend more $$ on special tools, machine feet;
first class -instructor had us cut up all that fabulous fabric! and a king sized batt---then gave us a 'homework sheet' and sent us out the door- i did my homework-which was starting to sandwich/decorative stitch a set of (logs)
next class comes along---and the instructor called in sick ---1/2 an hour after class start time- we were there-set up-waiting---
then a week later she started a new job-and said- sorry i won't be able to do this class!!!
the quilt shop was (almost) as upset as the 4 students---and did give us our class fees back---but that did not change the fact i have a large box of cut up fabric---sometimes i pull out the pattern and start reading it---but it is overwhelming- and i don't know where i am- or ...oh lots of things---
my hubby says i need to clear up my space---sit down and just do that quilt---with nothing else going on around me- he's right---but then another (deterent) is i have no use for the quilt= i do not even know anyone who would want a quilt of silks/orientals- and have so much $$ already in it---who would want to buy it? and what if i could not get permission to sell it----so many things---see- i overwhelm myself with this project---i did pull it out of the closet- the box is sitting under my ironing board- in clear site---poking me---maybe this will be the year...
sorry for getting so (windy) about it- guess it is a passionate topic for me right now