It’s one of the great truisms of IT: “Wait for Service Pack 1.”

Through the years, that’s been consistently excellent advice. It’s not so much dogma as common sense. Unless there’s a compelling business advantage to deploying a brand-new release, you’ll suffer less downtime and require fewer antacids if you wait and watch as other people discover the places where it crashes or falls short.

Historically, Service Pack 1 for any Windows release has been an official declaration that Microsoft believes it’s found, fixed, and documented all the bugs in the original release. It’s the all-clear signal for conservative IT folks to begin thinking about deploying the new release into the production network.

So what are we to do in the “Windows as a service” era? That’s the description Microsoft executives like to use when describing the development process of Windows 10. There are no service packs–security fixes and new features show up side by side, when they’re ready.

If there’s no Service Pack 1, how do you know how long to wait?

I’m not sure anyone really knows the answer to that question. This is uncharted territory for Microsoft and its customers. The company has done a good job of proving that it can deliver regular updates to services like Azure and Office 365, but deploying code in a data center is different from building a client operating system compatible with an infinite number of hardware variations.

We won’t have a new set of guidelines until we see how well Microsoft does with this new approach, but we can look at past Windows releases to see how long it took to get from RTM to Service Pack 1.

For Windows Vista, the wait for SP1 was almost exactly 15 months. The first (and only) service pack for Windows 7 arrived 19 months after its RTM date. Windows 8.1, which included significant new features as well as bug fixes, was released a little more than 13 months after Windows 8.

In other words, the “Wait for Service Pack 1” benchmark translates to about 15 months, on average. And that’s exactly the evaluation period I recommend for businesses planning a Windows 10 deployment.

The first three months after the July 29 launch date will bring a mad scramble to fix bugs and finish a few features in Windows 10.

Yes, that’s right–on launch day, Windows 10 will not be feature complete. The biggest missing piece is the new unified sync client for OneDrive (the consumer cloud storage service) and OneDrive for Business (the cloud storage service for business Office 365 subscriptions). There are other odds and ends as well, including the new Microsoft Edge browser, which has so far appeared in preview editions only under its code name, Project Spartan.

I expect that a lot of those post-launch pieces will be wrapped up by late October, in time for Windows 10 PCs to hit the retail channel for the holiday season.

And then the acid test begins: Can Microsoft continue to improve the reliability and functionality of Windows 10 at a steady cadence over the following year?

The new update cadence is likely to be a bit irregular, but you can probably get a good idea of how the entire project is progressing if you check in every three months or so. Even with rough edges and some incomplete features, Windows 10 should be stable and usable during that time. If you have employees or team members who want to trial the new operating system, encourage them to do so. There’s nothing like real world usage of your in-house apps and services to help you make decisions.

By the beginning of summer next year, we should have a pretty good idea of whether Windows 10 has settled into a steady, predictable rhythm. And that might be time to break out the spreadsheets and begin building a schedule for your migration.