Some quilts take a l-o-n-g time to what I call "mature". I rarely work from commercial patterns, so it takes me a long time to figure out design, fabric, borders and then the quilting pattern...It can take years. So of course I'm working on other things at the same time.
Others are spur of the moment and I can design it, piece it and quilt it in an afternoon.
Sometimes, I get a little obsessed and I'll spend days piecing a quilt straight through and then quilting it and binding it and it's done in a week.
Or, I can spend months working on a fabric collage for a quilt and not working on anything else.
If I know I have a deadline, like a baby quilt I start way ahead of time because I don't do great with deadlines.
Watson