The quick but rough way to answer this is to log in to the app, go to the "Policy" tab and apply any filters you're interested in (government documents only, from South America, central banks only etc.) and then run a search.
The "Year" filter on the left hand side of the screen will then show you what the distribution of articles by year is (the screenshot below is for the whole database and was taken in March 2020).
The longer, more complete answer is that it is source dependent.
Overton tries to collect as many documents as it can from each policy source. There are documents from the Canadian government from as far back as the 1920s in the system.
But government departments are closed down or merged, think tanks shuttered and websites redesigned and usually there are only a subset of documents still available.
Overton started collecting data in 2019 and our rule of thumb is that the coverage is generally good from 2015 onwards and sparse before 2009 - though remember that this is a generalization.