It'll be possible to delete your Google Duo account soon as the most recent version of the application which is version 24, has revealed some information about upcoming changes that it looks like Google is getting ready to implement, though it is worth pointing out that there is no indication of when exactly these changes will be made or when they will go live for users, and whether or not they will need to be part of an update to the app or if Google will simply be able to enable things server side.
In regards to the account deletion feature, this isn't something that is necessarily going to be a new option for users to interact with. It's already possible to unregister your phone number from Duo so it's like you're starting fresh or if you simply decide to stop using it, and based on the details it seems that Google is going to potentially be replacing the "unregister phone number" option with the delete your account option, which might be easier for people to understand as they'll be able to more easily correlate this option with stopping the usage of the app. Naturally, if you choose to delete your Duo account you'll be greeted with a little popup message that notifies you of what this will do, which in this case would be preventing you from making or receiving calls through Google Duo.
Another change comes in the form of a more fleshed out Google account linking page within the application. Linking your Google account isn't possible just yet as the option hasn't been added to the app, with Google still continuing development on the feature before it pushes the option out through a future update. For now it looks like Google is adding a new splash screen to the page that comes up when you'll select this option, with a button that you can tap that says "later" if you're not interested in linking your Google account right away and a button that says "yes, I'm in" if you wish to proceed in which case you should be taken to a screen to link your account with Duo.