I was in St James Catholic church in Arlington Heights last weekend. Here is the setup. That wall is 27 feet away from me. I was bouncing my SB900 speedlight off of it to give me a direction of light.
In this shot, you can see the existing light, Exposure was F4, 1/60, ISO 1600.
In this shot you can see what the flash added. Settings were the same as above.
Even though the wall was almost 30 feet away, enough light returned to brighten the image to proper exposure.