How long should the prologue to a novel be?

A short as possible and preferably absent all together. One of the most important things that you can do in a novel is to immediately provide a narrative hook. That is 'get the reader interested in the plot as quickly as possible'. Having a Preface only serves to delay that most important element. Of course you can make the preface as interesting as possible. In Jurassic Park we have a preface (prolong) in which an unseen velociraptor is tortured and kills a man, interesting in itself.