Each project takes the amount of time it takes... I know it can take me roughly one month of work start to finish on a queen sized quilt. Sometimes that means yes, 4 weeks at 40 hours a week for 160 hours. Other times it is less hours but still a month in time. And sometimes I might have something "in progress" for a long time without considering it a UFO.
Once I've declared it a UFO instead of a WIP, I'm usually motivated to git 'er done. I've considered my Barn Quilt project as on-going for about 2 years now. The reality is I just need to get myself to paper piece the blocks I want to paper piece, and I haven't wanted to.
I consider a top as a finished piece, just not quite a quilt yet. Since my favorite parts of the process are the 3 P's (planning, playing with fabric, piecing) it might take years before a quilt is quilted down, but it is stored together with it's backing and binding.