Normally speaking, although the instructions provided by the visa post often ask for police certificates and medicals to be given at the very beginning, as a lawyer I would normally advise them to wait until the application is about to be approved before submitting these documents. I think there have to be better instructions in the packages themselves for applicants, saying to wait for the police certificates and medicals until a certain period of time because you're right: they absolutely do expire after a year and a half.
On November 3rd, 2009. See this statement in context.