Microsoft licenses are, to say the least, somewhat dubious. I would imagine OEM are timeless, limited only by the lifespan of the OS. The thing is, if you install Windows on a "new computer", which practically means a new mobo - new memories are of no concern, neither a new CPU or GPU, or these combined.
In full versions, like I have 7 Pro, they give 20 downloads. Then again, having stopped the sell of Win 7, what does it mean?
All definite seems to be so very relative?