As said before it all depends on the snowfall load and the roofing material. Here in the Heart of Dixie we might get a dusting of snow each year and I use standing ridge formed metal roofing so my roof drop will be miminal. I have some 16' wide lean to's on the sides of my barn that drops only 6" or so.
Bottom line, you have to consider the drop for your area and the materials you will use on the roof.