I read all sorts of instructions on doing this and lots of them said to let the items sit on the construction paper for 4-6 hours, one even said to tape the items down and put the paper in the window for a week! We got out prints in 1.5 hours and it wasn't even at high noon. It was from 2:30-4pm on a sunny day. That's it.
If you want to make more of a science experiment out of it, you could use the same color of paper and put them out at staggered times, but end them all at the same time.
We just used items around our house that could lay flat on the paper and wouldn't blow away in a slight wind.
My daughter plans to use her sun print paper as the cover to a book she wants to create, I am using mine as a card.