What is the usual interval between filming and releasing a movie [closed]

It depends.

Obviously different movies require different amounts of post-production. And different movies follow different schedules. For example, principle photography for Return of the King was shot as part of a gigantic block of filming the trilogy in 1999/2000 - three years before that movie's release).

That said, someone has crunched the numbers. The average movie takes 871 days between the studio announcing the project and it hitting the cinemas, with 301 days for post-production (based on 782 live action movies made between 2006 and 2016).

As to how that compares historically? That's proving tricky to find. Obviously very early movies, say 1920s and 1930s, mostly would have had shorter post-production times. My feeling, though, is that by the late 1940s and certainly the 1950s, your typical movie schedule wouldn't be that different, on average.

As a bonus, Wikipedia has a list of movies with the longest production times. Many of these are animations, which naturally take longer than live action, others are indie projects like Peter Jackson's Bad Taste, while others were ambitious/groundbreaking or simply in development hell. Some of these movies are also quite old ones, like Howard Hughes' Hell's Angels.