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Apple this morning released Mountain Lion, the next version of the Mac OS X operating system, as a $19.99 upgrade in the Mac App Store.

Many of the most attention-grabbing features will be familiar to iPad and iPhone users, including a new Notification Center and Game Center, in addition to Twitter integration. But what’s particularly interesting from a competitive standpoint is the timing of this release.

Compared with the major changes in Windows 8, the upcoming version of Microsoft’s operating system, Mountain Lion is an incremental update. That reflects Apple’s strategy of releasing new versions on a shorter turnaround. The previous version, Lion, came out a year ago.

Microsoft is still on a three-year turnaround cycle for major new versions. You can debate whether service packs count as updates, but Windows 7 was the last major release of the operating system, and it came out in 2009.

Microsoft exec Steven Sinofsky makes the Windows 8 release announcement at an internal sales event in Atlanta.

Apple also has the advantage of owning its own hardware ecosystem. Microsoft expects to release Windows 8 to computer makers early next month, but the operating system won’t hit the market until October — a gap of nearly three months to give PC makers time to get machines with the new Windows on the market.

Microsoft has already put a toe in those waters with the unveiling of its own computer, the Surface tablet.

Here’s the next question: When will we start seeing yearly Windows releases?

One of the big hurdles to this would be Microsoft’s big enterprise focus. Depending on the degree of change, IT departments would tend balk at major new versions of Windows each year.

Simon Bisson and Mary Branscombe and Mary Jo Foley have also explored this topic. One theory is that the company could start with yearly updates of WindowsRT, the version for tablets running ARM processors.

Whatever the case, in this world of instant updates, three years is starting to seem like a very long time.

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