Microsoft might be happy to give free copies of Windows 10 away to anyone who asks, but it’s unlikely the operating system will be free to use forever. And although the company hasn’t admitted it’s planning to introduce some kind of subscription model for Windows, it’s looking more likely than ever that it will do exactly that, after revealing users will only enjoy free upgrades for a certain amount of time.

Exactly how long you’ll get free Windows 10 updates depends on the lifespan of your device, the company revealed in a PowerPoint presentation posted onto its investors website and first obtained by ComputerWorld‘s Gregg Keizer.

According to the documents, Windows 10 users will get free refreshes for between two and four years after they first install the software. That was revealed in a slide deck that contains additional information about how Microsoft plans to defer revenues for Windows 10.

“Revenue allocated is deferred and recognized on a straight-line basis over the estimated period the software upgrades are expected to be provided by estimated device life,” one slide says. “[The estimated device life] can range from two to four years.”

Microsoft says that device lifetimes (and thus, the free support period) will be determined according to “customer type” without elaborating any further. ComputerWorld’s Keizer suggests this means separating consumers and business customers, saying “Microsoft may use the edition of Windows 10 running on the device to make that determination. If that’s the case, free upgrades for Windows 10 Home, say, could be different — delivered for a shorter stretch, perhaps — than for Windows 10 Pro, the more expensive SKU (stock-keeping unit) aimed at power users, small businesses and corporations.”

That could be bad news for consumers, because Microsoft has said nothing about what happens once a device’s lifetime has expired. The obvious assumption is that consumers would have to pay for further upgrades after two years, with the alternatives (no more upgrades, or a brand new “Windows 11” both seemingly highly unlikely at this stage).

No matter whether they’re free or not, the Windows updates will be more crucial than ever for users, because many analysts believe Microsoft’s ultimate strategy is to transform Windows into an operating system “as-a-service”. A slide in the presentation underscores this point clearly enough, saying “With the launch of Windows 10, Microsoft will provide new features and functionality over time,” while another slide adds that “We will continue to keep it current for the supported lifetime of the device. We think of Windows as a Service — continuous updates over time.”

The implications are clear enough. Microsoft won’t be pushing free updates forever, and if you don’t continue to get hold of those updates somehow (most likely by paying a fee) you’re quickly going to be left behind, running an outdated and eventually insecure operating system.

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