Default / Recommended CiviCRM Extension Directory

I wanted to create a Composer Installer for CiviCRM Extensions:
https://github.com/composer/installers

However, I need to know what the default / recommended Extension directory is.

I found this page explaining that the user needs to set it:
http://wiki.civicrm.org/confluence/display/CRMDOC42/Extensions

However, I didn't see a default or a recommended.

Of course, with Composer installers, users are able to override the directory:
https://github.com/composer/installers#custom-install-paths

But I'd like to provide a default for users.

Thanks!