I'll always remember the time I was sitting on the front porch of my college roommate's parent's house. In north central KS.
It started just before dusk, I was looking south with an uninterrupted view of the horizon. I watched a storm move west to east for at least an hour until it was too dark to see it well. I have no idea how far away it was, but I could see the entire storm cloud formation from that distance.