Provided you have an internet connection, Descript will automatically save your Project as you work - there's no need to manually save. If you have an internet connection, Descript will automatically save changes to the Cloud. If you're offline, Descript will save changes locally and automatically save them to the Cloud the next time you open the app with an internet connection.
You can verify that Descript has recently saved your work by moving your mouse over the Cloud icon in the upper-right corner of the app; a panel will appear indicating the last time the Project was successfully saved.
If you notice that the Cloud icon has a small spinning circle icon like the one below, that means that the application is currently syncing your work to Descript Drive (there's no need to wait for it to finish — you can keep working).