Indoor plumbing has been around since the year 1892 when it was first used in the Tremont Hotel of Boston which was the first hotel of its kind to feature indoor plumbing for guests.
Then in 1930s Rural America started getting indoor plumbing in their homes however some people in way off rural areas didn't have indoor plumbing until the 1940s.
However most people did have indoor plumbing in their homes around the year 1930.