It's generally safer to go with 20 years as the mark for patent expiration.
Patents have different durations of protection depending on the type of
patent but 20 years is the max so just go with that.
Images displayed in a patent can still have individual copyright protection.
Don't ever make the mistake of assuming that since a patent is expired, the
images in that patent are expired as well. This is not the case, normal
copyright rules and durations still apply. However, in most cases, the
images found in patents lack the one crucial ingredient that would qualify
them for copyright protection - an actual copyright notice. If a copyright
notice is in place, research would have to be done on the image to find out
if the copyright is still in force.
If the image does NOT have a copyright notice, it would still need to have
been produced before 1978 in order to be in public domain.
So, the quickest and easiest way to handle images found in patents (in order
to make sure they are safe to use) is to...
1) only deal with patents produced before 1978 (this way you're sure the
patent is actually expired AND the patents are in the age range where no
copyright notice on an image places it in the public domain).
2) make sure there in no copyright notice on the image
Throw everything else out the window.