How fast can the human eye see?

I'm a big fan of googling instead of being lazy and having someone else do it for me, but while searching for the answer to my question and saw "Get Pregnant Fast using Lunar Fertility" was one of the hits i got off of google I realized googling might not be the answer for this one.

My question is: How fast would an event have to occur for the human eye to miss it? If that was hard to understand, I'm wondering how fast a flash of light, or a movement would have to happen so that the eye couldn't see it, or couldn't send a signal to the brain.

I hope you understand what I'm asking, and I hope someone has the answer.