But a job posting at the Microsoft Careers page has revealed that the major updates will continue on a yearly schedule: "This Senior Program Manager position will be the CXE [customer experience engineering] Feature PM that drives all development work on the Application Platform for update releases between major yearly releases."
Windows Phone 7 arrived in October of 2010, and then received the minor NoDo update. And at some point in the Fall, we will receive the massive Mango update. So we could have inferred that it was a yearly schedule. But it's nice to have confirmation.
Do you think a yearly update schedule is sufficient? As Microsoft is playing catch-up with iOS and Android, perhaps a semi-annual schedule should be in place. That wouldn't necessarily hasten their development, but it might increase customer satisfaction if they feel like WP7 is progressing more quickly.
source: Microsoft via WMPoweruser