Microsoft has not yet defined that lifetime.
But in a slide shown to analysts last week, Microsoft said that the lifetime of the deferral — and thus the lifetime of a device equipped with Windows 10 — will be based on the device type. "Estimated useful lives will be determined by form factor," the slide stated. "As a result deferral periods may vary."
Microsoft will probably set the support in years, but one kind of device will not necessarily have the same lifespan as another. A tablet, for instance, could top with a two-year life, while a notebook could be pegged at four years and a 2-in-1 device at three. Or Microsoft could define form factor by screen size, as it already does to some degree.
It's unclear exactly how the supported lifetimes and associated deferrals will affect customers: Microsoft has said nothing about what happens after the lifetime expires, including whether upgrades would be discontinued entirely or would be available for a fee.
It's possible, then, that after, say, four years — if Microsoft defines a notebook's lifetime at that length — Windows 10 will no longer be automatically upgraded for free.
Hood also promised Wall Street that she would detail the deferrals later this year. "We'll have a conference call much like we've done with other transitions as we get closer to the impact to share the exact details on lifecycles, how long the time will be, and the exact impact we expect," Hood said. She also pledged that the company would provide what she called a "comparability bridge" in its earnings statements, meaning that those statements would show last year's revenue as if Windows revenue had been deferred so that analysts will have an apples-to-apples comparison.
Microsoft held a similar one-time call for analysts in September 2013, near the end of the quarter when former CEO Steve Ballmer launched a major corporate restructuring that in turn required a revamp of its financial reporting.
If Microsoft starts selling Windows 10 licenses to OEMs this quarter, it would probably announce the support lifetimes in June, several weeks before its Q2 earnings call. Likewise, if OEM sales began in or after July, it would disclose that information in September.
Financially, the deferral will reduce Windows revenue on Microsoft's balance sheet, at least for a time, as the income once recognized during the quarter of sale is instead split amongst multiple quarters or years.
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