This question pertains to the pages router. Is there any reason for SSG/ISR generated pages to expire and no longer be cached, so that the page would have to generate again next time it's requested? Other than having a revalidation period on ISR pages of course.

For example, is there a limit to the number of generated pages that are kept in the cache, so for example if your build generates 15,000 SSG pages, and then ISR page requests total another 15,000 over a period of time, pages will expire from the cache after a certain limit (like 20,000 pages total) as the generated ISR pages accumulate and add to the total?

Or is there a limit to the time for pages to be cached, so that SSG or ISR pages will expire from the cache after a certain time limit (like 30 days with no requests for the page)?

Or any other reason whatsoever? I'm trying to get full clarity on whether SSG or ISR pages ever expire for any reason, or if they're kept as static generated pages forever (until the next build).