I have one WIP that was started almost 20 years ago. I will get it done, one of these days.
Other quilts I can knock out in less than a week.
There's room for both extremes.
And I personally do not have a rule that one thing has to be finished before I start another thing. For me, that would really stifle my enjoyment of quilting. Sometimes my brain is in planning and designing mode, and other times it's more into piecing or finishing. I like to have options.
RST