I over lapped 12 inches on both ends.
I raised my roof 24 inches. This made my splice pieces 48 inches long, making best use of the 4x8 sheets of steel we sheered to form the ribs from with no waste.
I would not do less, you want to spread to connection over as large area as you can.
Mine were custom formed 14ga ribs made to the same shape as the original ribs.
I was the first one I ever seen to do the custom formed ribs. It's the best and strongest method that can be used.
I was also the first to use scaffold to lift the roof. I found it vary safe, simple and easy.
More can be seen here in my latest build thread.