If you are talking about the technology known as sprite sheets: Such a technology and method is not patentable, as it is not novel to put images in rows or cells. You could easily do this yourself or use countless other softwares to do this without fear of legal issues.
If you are talking about the art that someone created that is a sprite sheet: all art is copyrighted at its creation, and unless you are given license to use said work, you would be absolutely infringing upon the lawful intellectual property owners rights.
As for "how long" such a copyright lasts, that depends on the owner of the copyright, the date of creation, and many other factors. For instance:
"For works published after 1977, the copyright lasts for the life of
the author plus 70 years. However, if the work is a work for hire
(that is, the work is done in the course of employment or has been
specifically commissioned) or is published anonymously or under a
pseudonym, the copyright lasts between 95 and 120 years, depending on
the date the work is published. All works published in the United
States before 1923 are in the public domain. Works published after
1922, but before 1978 are protected for 95 years from the date of
publication. If the work was created, but not published, before 1978,
the copyright lasts for the life of the author plus 70 years. However,
even if the author died over 70 years ago, the copyright in an
unpublished work lasts until December 31, 2002. And if such a work is
published before December 31, 2002, the copyright will last until
December 31, 2047."
[Excerpt from: https://fairuse.stanford.edu/overview/faqs/copyright-basics/#how_long_does_a_copyright_last ]
In any case, if you want to use something that you didn't make (even to edit it slightly and make it better) you really should ask the owner of the ip for written permission to do so; not only to get their permission but to protect your butt from losing rights to use your creation that used their IP (a spit and a handshake: "yeah, go ahead" said in person or over the phone isn't enough... get an email or letter stating unarguably that you are allowed to use the ip in question).