They tend to sit there for 30 days before they disappear. I don't know if the US banking system (where I assume you are) than those in Europe and Australia, but once they're there, I've never seen them disappear before 30 days (but never beyond 30 days either - 30 days seem to be the cut off). Sometimes I have 10+ pre-authorisations from the previous 30 days amounting to many thousands of Euros sitting there.