Is it any unwritten rule, that for how long after getting PhD is it appropriate to publish with your advisor? After reading this question on What is wrong with continuing to publish with your PhD adviser I wonder whether there is something like an "unwritten rule" for how long after getting your PhD is it suitable to publish with your supervisor. 

Is there some sort of, let's say "secret statistic" related to the ratio of your own papers vs. your papers with your supervisor, upon you are rated among fellow scientist long after your graduation?
You should not only publish with your advisor, because it suggests a lack of independence, and that rather than emerging as an independent researcher, you are still essentially "doing the work" for your advisor's ideas.

That being said, I know a number of people who still work with their advisors on projects - the key is to project an image of "long-standing partnership". This is especially compounded in public health (my field) where a single large study is a vast mine of potential papers, and so it's likely someone will be working on the same data with the same people long after they've started their own careers. The key is to have a place carved out that's clearly "Yours".

If there's a secret statistic for it, I must not have gotten my copy in the mail yet.