said 10 months, 3 weeks ago:
year can change based on the item in question, but things usually enter the public domain at 50 or 75 years old. I read that Peter Pan will not enter the public domain as long as a certain hospital exists (which profits from the revenue Peter brings) and a few others. An artist can also forfeit her rights and put something into the public domain.
SO, you are (probably) safe using something 75 years old, but verify anything newer.