Sistine Chapel was 4-5 years I think.
Might not fully apply, but a different perspective. A skyscraper can take a minimum of 5 years to build (so a smaller variety). And you're dealing with bureaucracy with that. But take the WTC/Freedom Tower as an example. It started to be rebuilt in late 2006. And was finished in mid-2013. So, 7-years, I guess. And that's got a ton of moving parts.
Meanwhile, these people will say it's about quality. Never mind the fact that they used to do be able to do 20 page comics in maybe a month? And most of these campaigns seem to hover around 40-50 pages comics. Sure, without a deadline, they can make them a little better. And I get it. Life happens. You have to deal with other people's schedules. Maybe there is an issue with the printer. But there comes a point where it looks like you're just goofing off or have major OCD. Or possibly just not really interested in the project you started. Particularly when getting them to give an update is like pulling teeth.
Having said that, I think a year is fair. 6 months is better.