RANDY CALIFORNIA’S ESTATE COULDN’T PROVE THAT JIMMY PAGE STOLE THE OPENING CHORD PROGRESSION FOR “STAIRWAY”

Apparently it’s tough to prove that if someone else used the same chord progression in their song that you used in yours, they must have copied it from you.  That’s my takeaway from the verdict in the Led Zeppelin copyright case, announced today. 
I can’t help wondering, though: how much time do you have to make a claim for copyright infringement?  Stairway to Heaven was released in 1971, the guy whose estate is claiming copyright infringement died in 1997, and the lawsuit wasn’t filed until 2014.  I’m sure our intellectual property guru could explain how the lawsuit wasn’t barred by a statute of limitations, but I can’t.