Same as PR application, you need to provide a police certificate for every stay of more than 180 days. The certificate cna be old as it doesn't matter as it verifies no police records during that time period. If you happen to go there again say after an year for another 180 days, then you may ask them for verification for the two said time periods. If you already have obtained one for the previous stint, then you may just ask for the latter period and attach two certificates. Crux, age of certificate doesn't matters but it should match the dates that you have mentioned in your application. Hope it helps. Plus as other member pointed out, no need to provide any certificates for stays beyond 4 years prior to your application.