It is recommended to update or (re)publish your app every six month. Click here to go to the related webpage. To be honest, I do not know if this it true for SharePoint forms (I assume it is), but for apps it is recommended.
So how does one know which apps are not updated or (re)published in six months? Well, PowerShell to the rescue!
There are PowerShell cmdlets for PowerApps (and Flow) and a blog post about this was published on 2018-05-29. Click here for the blog post. Some examples are given in that blog post but none which can directly be used for this overview.
To get a list of apps which have not been updated or (re)published in six months (in this case 150 days), just execute the following code: