I am not sure the statistics of this sort actually is available, exactly because the first non-Para IV generic is [almost] always launched the next day after the patent expires. For any publicly owned Pharma company analysts very visibly forecast the genericization dates for their drugs - and I don't remember any of these predictions to be proven too early due to slow FDA action... Usually, approved ANDAs for a meaningful drug are lined up, and product is ready to be shipped long before the Orange Book dates.
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