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How often is too often?
I doubt that there is an answer to that question.
Remember that the objective of a search engine is to return a list of websites that "should" provide information relevant to a particular query.
One of the factors that goes into determining how frequently your site is re-indexed is how frequently it changes (this can be determined by the spider over time), and then a decision has to made as to whether your content remains valid for a particular query.
One of those factors could well be Page Rank - a site with high page rank that changes frequently is far less likely to be penalized for changing frequently than a site with little or no page rank.
So it's a resounding "it depends" i'm afraid.
BBC News for example changes several times a day, but is a high PR site and not penalized in any way at all.