for me, I would probably finish a 2 page layout every 90 - 120 min (if I'm journeling, embelishing, stamping, etc) - so with 15 hrs - I'd leave 1 hr for eating, breaks, shopping, and 14 hrs of crop time - I'd get 6 - 9 2 page layouts done, or about 12 - 18 pages. So I'd probably plan to have about 20 - 24 pages kitted.
Also, when I kit, I already know my layout, and have ordered my pictures sized for my layout to start with. I have my paper, stamps, main embelishments, etc all planned out. It just takes me a while to pull it together the way I want it (esp. if I'm inking paper edges, completing stitching on a page, etc).