There is no set date for features to move from beta to GA, it can take days, it can take months. Your best bet is to check the Roadmap and keep an eye on the Graph API changelog and the relevant MS blogs for announcements. If the feature has been previously publicly announced, chances are another post will follow up when it reaches GA, and you can always probe the post author for updates. Not all features are released in such way unfortunately.
When will the beta version of the MS Graph APIs be released into V1.0?
The beta version of the MS Graph APIs includes the ability to convert EML and MSG files to PDF. This feature can satisfy an enterprise capability for my organisation once it becomes available as an official Microsoft offering. However, I can find no indication on when this feature will be released from Beta. Where can I find this information?