The most recent 60-Second Science podcast summarizes a paper in Science magazine reporting that the average interval between a drug being deemed “promising” and the first paper appearing showing clinical effectiveness is 24 years.
Note that the publication of a paper saying a drug is clinically effective is a far cry from regulatory approval. Many new drugs that look like an improvement after a phase II trial turn out to be no better than existing treatments, and those really are better take years to achieve regulatory approval.
* * *