I wouldn't think there a large number of customers deciding "this is too unreliable, I'm leaving" on the basis of a few hours of down time. On the other hand,

I wouldn't think there a large number of customers deciding "this is too unreliable, I'm leaving" on the basis of a few hours of down time. On the other hand, there might be a large number of people who, upon finding your site down, decide to visit alternatives that are up at the time, and some of those people might decide they like the alternatives better.