We still don't have a general release date for Windows 8.1, but Microsoft has told its hardware partners that the final version will be available in late August. That means machines running Windows 8.1 will be out in plenty of time for Christmas, and we could see the final version for existing users as early as September.
Microsoft announced the late August release date at its Worldwide Partner Conference in Houston. Since Windows 8.1 will be a free upgrade that will be released via the built-in Windows store, there shouldn't need to be a lengthy delay between the code being finalised and it being made available for existing Windows 8 users. Producing boxed copies for people upgrading from earlier versions will take longer, though Microsoft has been emphasising downloads over store-bought physical discs.
In enterprise environments, upgrading is likely to be a slower process; while Windows XP officially ends all support next year, many sites are shifting to Windows 7 rather than 8 to avoid having to retrain staff so extensively. At the conference, Microsoft said more than 20 million "enterprise evaluations" of Windows 8 have taken place.
Windows at WPC 2013 [Windows Blog]