Hello Rob1961, thanks for posting on the Community page, 

There is no exact date when this will be released however The 'Get the desktop app' option in the Company Cog, will appear for customers using QuickBooks Online Accountants and QuickBooks Online Advanced in the next few days for them to download and start using the app.

Customers on Simple Start, Essentials and Plus cannot install the QuickBooks Online app , it will only  be available to QuickBooks Advanced and Accountants in the gears icon.