I believe the bottom line is that as long as your PC hardware does not significantly change, then the upgrade will not expire in any way in the near future. As Aura says the meaning of "Supported lifetime of the PC" is vague and does really tell us what will cause the upgrade to finally expire (however it does not look like there is a clock ticking a countdown unknown at this time - I suspect this is more likely to be related to the supply of drivers for various components, as hardware standards evolve, or to attrition of your hardware due to hardware failure/repair)
What is clear is that the ability to activate Windows after a significant hardware change could effectively expire the upgrade. In the past, you had a product key to identify your licence, and if it would not activate (as the fingerprint of your hardware had changed) then doing a telephone activation and explain the circumstances to the Microsoft Activation could get you running again.
With the Windows 10 promotional upgrade that model changes. You do not get a unique product key to identify your licence. When you perform the in-place upgrade, Microsoft add the fingerprint of your PC hardware in a list of PCs licensed to run Windows 10. If you attempt to activate Windows 10 on that same hardware again (even an entirely fresh install) it will activate as MS know that hardware fingerprint is licensed. That ability to activate does not expire at the end of one year - you could do a fresh install in two years time and it would activate - as long as the hardware fingerprint matches. If the fingerprint did not match then there is no way to tie the hardware to the upgraded licence, and you would not be able to activate Windows without buying a retail key.
So what is has hardware "fingerprint" I've been talking about? Microsoft do not release information about the exact things that it takes into account, but it is a fair bet to think that it is similar to earlier Windows versions. In that model, characteristics of your PC hardware are checked (component types and serial numbers) of those components) and if that least 7 out of those (about 11?) criteria continue to match after a hardware change, then activation is not affected (or a reactivation request will be allowed). The issue is that to many of these things are likely to change if your motherboard changes (CPU type, CPU serial number, NIC MAC address, IDE controller type) In that case - Windows would consider that is in new hardware and would not reactivate - It's possible that you could sweet talk an MS Activation telephone operators into completing activation, but that's not guaranteed. It may be they say they cannot help and your only recourse would be to buy a new key. In the past, MS have accepted genuine Motherboard replacements and manually reactivated Windows over the phone, however with Windows 10, there is no longer the lure of new Windows versions to drive sales, therefore they could choose to be more aggressive about these situations refuse activation as the hardware is different.
There has been a lot of speculation as to exactly how this will pan out, and no firm information from Microsoft as to what they will actually allow. Their official line seems to be however that if the hardware changes, that invalidates the licence. How strict they are with that remains to be seen. Indeed - that is a reason why I have only upgraded one out of three eligible systems that I have (a fixed configuration laptop). Two other homebuilt systems remain on their current OSs (Win 7 Ultimate and Win 8.1 pro).
One thing I did do (both on the laptop and on one of the homebuilt systems) was to, before committing to the upgrade, remove the boot HDD, replace it with a fresh drive, and perform a fresh install of Win 10 so that I could check driver and software compatibility. I did not enter a product key or attempt to activate the software - I used it just as a method to evaluate the final version of win 10 in order to check that vital software would work, and that all necessary drivers were available and operated correctly. Then went on to replace the Laptop HDD back with the original disk, performed the upgrade (to get the hardware fingerprint licences at Microsoft) then did a fresh install of Win 10 and reinstalled software (that went all OK). In the homebuilt system, the test install was OK, but I have chosen not to risk my OS licence on the lifetime of the the hardware, so I re-fitted the original HDD am continuing to run win 8.1 on it.