Real world support periods are shorter than they look
Here's something that's important to remember when you're talking about vendor support periods (eg, how long a vendor will support an OS release): real world support periods are always shorter than what the vendor gives you.
This isn't because the vendor is lying to you or is going to shorten the time. Instead it is because you can never really use the full support period; you'll never deploy a new version immediately after the vendor releases it, and you'll almost never run a version right up until its official end of life point. So in practice, you get some amount less of support time on either end; how much time you lose depends on how complicated building, testing and deployment is, and how much you push your margins for error.
(It's worth noting that you may also have scheduling issues that constrain when you can change software, which force you to change 'early'. For example, here at a university people react very badly to operating system upgrades at the start of terms, and the ideal time is during term breaks.)
Vendors can make this worse or better depending on how they overlap their releases and their support periods. No overlap at all is the worst, where you must be running the current version to get support, but fortunately this is not very common (perhaps because vendors worked out that it makes them intensely unpopular).
As a practical example, our build and test period for new operating system releases on our user machines tends to be on the order of six months (admittedly, this is partly because we don't push hard on it). This consists of rebuilding our install environment to set up the new Ubuntu LTS release, finding out and figuring out how to deal with all of the things that Ubuntu changed this time around, testing them ourselves, persuading users to test them (there's nothing quite like real users to find lingering issues), and then starting to push people to migrate. With bad timing and higher priority work preventing us from starting right away after the next Ubuntu LTS is released, this could actually cost us close to a year of theoretical support period (since we have upgrade times we have to avoid).
(Ubuntu LTS is supported for a sufficiently long time that this doesn't worry us, plus there is a substantial overlap of support periods from one LTS release to the next.)