For an early example, differential cryptanalysis methods were 'discovered' in late 1980s by Biham and Shamir; but it later turned out that resistance to it was pushed as a design consideration already back in 1974 when DES was designed, so NSA knew of it at least then, that's more than a decade.
We know that British intelligence (who don't have as much resources as NSA) had developed the RSA equivalent something like 5 years before Rivest/Shamir/Adleman got to it; and we still have no idea how far NSA was with that math at the time - all we have is circumstancial evidence such as travel reports of NSA representatives going to cryptography conferences and being satisfied that absolutely no math that's new (to them) or even potentially leading to something new was being revealed there.
We also have NSA suddenly changing recommendations to use/stop using certain cryptosystems that still doesn't make sufficient sense - e.g. the 2015 turning away from 'suite B' ECC may have been due to some quantum discovery as is claimed, or some other weakness being found, but it's been five years and we (as far as I understand) still don't know as much as they did back in 2015, so they're more than 5 years ahead. But to know whether the current advantage is ten years or more or less, we'll have to wait a generation or so, it takes a long time for truth to leak.
We know that British intelligence (who don't have as much resources as NSA) had developed the RSA equivalent something like 5 years before Rivest/Shamir/Adleman got to it; and we still have no idea how far NSA was with that math at the time - all we have is circumstancial evidence such as travel reports of NSA representatives going to cryptography conferences and being satisfied that absolutely no math that's new (to them) or even potentially leading to something new was being revealed there.
We also have NSA suddenly changing recommendations to use/stop using certain cryptosystems that still doesn't make sufficient sense - e.g. the 2015 turning away from 'suite B' ECC may have been due to some quantum discovery as is claimed, or some other weakness being found, but it's been five years and we (as far as I understand) still don't know as much as they did back in 2015, so they're more than 5 years ahead. But to know whether the current advantage is ten years or more or less, we'll have to wait a generation or so, it takes a long time for truth to leak.