It's not a question of "how long". "Featured" does not mean what it means elsewhere, i.e. it's been chosen to be specially highlighted in some way.
A Featured Hub is one that has passed the QAP test and therefore is visible to the general public and can be found on Google and other search engines. Once it has passed the test, it will stay Featured, unless it fails to get traffic.
If your Hub fails the QAP test it is not Featured, it is made invisible to the search engines and to browsing visitors, and therefore will not earn any income.
Note that if you edit a Hub, it goes through the QAP process again.