scores upon scores of years

a score is 20 years

Yes, that's right. But how many years do you think "scores upon scores of years" mean? I'm thinking maybe hundreds of years. Or is it too long? Thanks.

As a purely technical issue Emily it'd have to be at least 40 years

My Better Half, who's much smarter than me, agrees. However, off the top of my head I'd consider anything up to 200 or maybe even 300 years 'cause more would then be "hundreds"

Scores is plural, therefore, just the use of the word scores implies 40 or more years. This expression, therefore, means at least 80 years, and is probably intended to imply many more.

Oops Emily, Set is right: 80 min