A score is a term used to refer to a period of 20 years. This usage comes from older English, particularly in historical contexts, where the word "score" was commonly used to denote a set of 20 items. For example, "four score and seven years ago" famously refers to 87 years (4 x 20 + 7) in Abraham Lincoln's Gettysburg Address.
So, a score is equivalent to 20 years.