After 10 minutes of searching on Wikipedia, i could find no reference to it even though I have heard of the law many times before.

Essentially the law of big numbers states that "A very large number multiplied by a small number is still a large number"

As the poster indicated, even a few minutes of daily span cleanup multiplied by millions of people equates to quite a bit of time.

It's large, not big :-) and it has a different meaning

http://en.wikipedia.org/wiki/Law_of_large_numbers