Is 11 - 12 inches of overhang on a shelf too much?
As long as you don't heavily load the shelf you should be fine. Make sure that you hit the studs when you install the shelf brackets. The brackets should have a load rating on them. Use properly sized screws and balance the load across the shelf placing heavier items above or between the brackets. If you get some flex in the shelf at the center you can always add a middle bracket using properly sized hollow-wall anchors.