The key has nothing to do with the DVD. If you have got a time limited product key for two years it will work for two years regardless if you install Windows from the ISO file you downloaded, from the additionally ordered DVD or from a DVD you borrowed from your friend. The DVD is only meant as a backup media in case you lose your downloaded ISO file.
If you for instance have a product key for Windows 7 Home Premium, it does not care from which install media you installed the Windows as long as the edition matches the key, in this example Home Premium. You can download Windows 7 Home premium from Microsoft Store and install using this key, or you can use my Home Premium DVD and install using this key, or you can go to a PC shop and buy Home premium DVD and install it using this key.
Kari