For the U.S.

"The reason that New Year’s Day 2019 has special significance arises from the 1976 changes in copyright law’s retroactive extensions. First, the 1976 law extended the 56-year period (28 plus an equal renewal) to 75 years. That meant work through 1922 was protected until 1998. Then, in 1998, the Sonny Bono Act also fixed a period of 95 years for anything placed under copyright from 1923 to 1977, after which the measure isn’t fixed, but based on when an author perishes. Hence the long gap from 1998 until now, and why the drought’s about to end. Of course, it’s never easy. If you published something between 1923 and 1963 and wanted to renew copyright, the law required registration with the U.S. Copyright Office at any point in the first 28 years of copyright, followed at the 28-year mark with the renewal request. Without both a registration and a renewal, anything between 1923 and 1963 is already in the public domain. Many books, songs, and other printed media were never renewed by the author or publisher due to lack of sales or interest, an author’s death, or a publisher’s shutting down or bankruptcy. One estimate from 2011 suggests about 90 percent of works published in the 1920s haven’t been renewed. That number shifts to 60 percent or so for works from the 1940s. But there are murky issues about ownership and other factors for as many as 30 percent of books from 1923 to 1963. It’s impossible to determine copyright status easily for them."

https://www.theatlantic.com/technology/archive/2018/04/copywritten-so-dont-copy-me/557420/