A: 15 years B: 20 years C: 25 years D: 30 years E: 40 years F: 50 years G: other
I’m almost afraid to answer this question.
The rock from the 60s and 70s got ubiquitously considered classic rock by the early 90s, but I wouldn’t classify anything beyond the early 80s as “classic”. I think that the genre may expand with time, but the 60s and 70s were the true origins of rock. The music past that is definitely not new, but not “classic”.
I guess that makes my answer G.
Edit: From a marketing perspective, I’m about to cringe hard, music up to the mid-90s is considered classic rock. To me that seems like it’s just an easy way to keep “classic rock” more entertaining and nostalgic for the older markets, and more relevant for the younger ones.
I hear bands like Linkin Park referred to as classic rock quite a bit and that’s early 2000’s. I cringe every time someone says it.
The thing is is that they’re not quite wrong.
It’s not quite right, but it will only become more true.
