How often do Chrome extensions automatically update?

Currently this defaults to 5 hours (based on the code here).

You can override this by launching chrome with the extensions-update-frequency command-line parameter, which is the frequency in seconds. And you can go to chrome://extensions, tick the Developer mode checkbox at the top right, then press the Update Extensions Now button

Chrome docs doesn't specify this 5 hour value though, so it could change in future versions without notice:

Every few hours, the browser checks whether any installed extensions or apps have an update URL. For each one, it makes a request to that URL looking for an update manifest XML file.

Restarting the browser and OS didn't help. Eventually, the extension updated itself, but it took at least 48 hours.