I have very limited experience, and it points to a wide range from a few days to few months. When I stumble about a 1y+ old flag, I tend to delete it (and the dead code path that it comes with).
What's your experience?
Other times integration with third-party tools was what held us back. We rewrote our product pages in 2013 but our recommendations vendor was scraping the old version until 2015 because no one wanted to spend the vendor hours switching it to the new version.
My favorite was our add-to-cart actions. In the old platform we ended up with about 10 different user flows after clicking the "Add to cart" button from 2013-2016. This case was driven by heavy AB testing (should we show a confirmation modal? tooltip? send them to the cart page? what about an interstitial page that shows recommended add-on products? etc). In this case we accepted the overhead of lots of feature switches because a .1% conversion bump moved the needle pretty far.
The shorter flags have lived for a couple months as we build a new feature and then test into it at small percentages to work the bugs out. Once they're in at 100% and we're confident we rip the flag out.
For the temporary type of toggle, which is what I was addressing with this blog post, my experience coincides with yours-- usually a few weeks.
The trick with deleting a year-old flag (which I was trying to address with this post) was that you need to be careful when deleting code that you haven't worked on in over a year. If you have the list of necessary changes all pre-baked in a branch, this can be at least a little easier.
For instance GCC has a feature flag called -ansi which gives you C90 compatibility.
C90 was superseded in 1999 by C99, and so that's 17 years of compatibility, and counting.