Windows 10's Redstone 3 update is going to arrive in September and Microsoft has confirmed it plans to release updates twice per year. Those releases are being planned for March and September of each year. Each will be supported for 18 months.
Redstone 3 preview builds have already begun showing up, though the Creators Update was only recently released. The Creators Update is still rolling out.
This same release schedule applies to Microsoft Office.
Based on this feedback, I am excited to share today that we are aligning the servicing models for Windows 10, Office 365 ProPlus and System Center Configuration Manager for our customers, particularly those with Secure Productive Enterprise. - Bernardo Caldas, Microsoft