I have a plugin in the wordpress.org repository and I have updated the plugin to a new version and that new version is reflected in the listing
Congrats!
but my sites have the previous version. How long does it take before my new version is available to wp sites as an upgrade available?
It should be immediate, but, .org doesn’t notify your site. Instead, your site checks .org for updates. You can force it by going to the updates page and checking again:
