Well, let's put it this way- I've been quilting for 15 years now and have only actually completed 3 quilts! lol
But that's primarily because my quilts tend to sit in the closet for VERY long times in between spurts of working on them. And I currently do have two that are at the quilting stage and three others that are in the piecing stage. Also, I do handquilt so that takes some time. The quilt I'm currently working on (full-size)was started about 10 years ago but I've done most of it in the last month and a half or so. I'm hoping to have it finished within the next few months.
So, like everyone else says- it depends on a lot!