as N4 noted, principle photog on WS was last summer. post started last fall.
Avatar was 4 years of post.
re: "Smaller budget, smaller cast, smaller movie shouldn't take this long."
unless the divisor (workforce) is smaller than you assume.
consider:
if you have 100 people on a 10,000 person-hour job, how many days will it take?
if you have 5 people on a 1000 person-hour job, how many days will it take?
ie: Cameron had hundreds of people working on Avatar.
How many people total work at Filmworks?
How many of those are dedicated to WS?