Faster than you think
The pictures below have pixels randomly colored either black or white. If you have Netscape 3 or Internet Explorer 3, you can see them change as you pass your mouse over them. Each picture has two versions: a random pattern, and a pattern with symmetry around a vertical central axis.
How long does it take to see symmetry? 1/2 second? 1/10second?
( QuickTime for Windows or Mac and QuickTime plugin required)