Yes, very few works need more than the original 7 years in order for them to get a sufficient return to make them economically viable.

I wonder if that applies to novels that don’t become bestsellers? A book that simply does okay. People buy it, never a ton of them, but always a few, every few days, for a long time. No idea what the answer is, just something that popped into my head.

When/where was it 7?

IIRC under the Statute of Anne, it was 7 (plus 7 if you paid again).

It seems like a good balance between private monopoly and public interest for the shirts of artistic works copyright protects.

Statue of Anne was 14+14. I'm not sure exactly if/how payment worked.