Yeah, it's definitely an expression and generally just means "a long time", but it's hilarious because it's literally 10,000 years. In isolation it wouldn't mean much - the "you still haven't changed" bit is much more telling, since it pretty clearly states prior familiarity. Even if you take "10,000 years" as just "a long time", that means something.

But "you've already repaid it" is even more telling, yes!