If Push notifications are set up correctly, all devices must be notified about the new version instantly (within a few seconds). After that, they need a minute or two to download and install the app.
There are few reasons why the new version may not be installed within a few minutes.
1. (Most probable) Push notifications are not working well, see details here: https://qa.h-mdm.com/5102/
2. The download (which is in the background) has failed.
Headwind MDM sends a request for changes in the configuration (and retries to download and install a new version) 4 times per day. This time can't be updated from the UI however can be adjusted in the Android application code.