Do humble bundle promotional keys expire?

Yes.

Yes, they do expire today (27th May).
Well, talking bout that:

$J9xx-MGYIJ-$79BP

$ is 6+2
xx = 8th letter of the alphabet

Most keys expire and if they do they will post the end date.I am not sure what happens to revealed keys after the date of expire passes.I do know I forgot to redeem one some time ago that expired and it removed the key.

So I am not 100% sure if you reveal the key it will be good after that or it will not be good regardless if you revealed it and did not use it.I suspect though that the keys will no longer be valid regardless unless used.I see no point in making keys expire if there is a way to avoid it from expiring.

I do not have a problem with this as it really does help keep from exploiting free keys later for profit or for hoarding for reselling and ect.While it does not prevent all of it nothing will it is a good solution imo.

On a side note if your friend has access to another computer they can install steam on that and redeem the key that way for future use.I always keep a pc as a backup stored away.It does not have to be anything fancy just good enough to access things like Steam.I know this is not viable option for all but just a thought.

Keys themselves shouldn't expire once revealed, or if they do, it's way later than the advertised end date.
However, their entries do vanish from your Humble library, so you need to reveal and store them beforehand if you wish to use them.
I can personally confirm that I could activate GRID, DiRT 3 Complete Edition, and DiRT Showdown on my brother's account, long after the corresponding promotions had ended, without any issue whatsoever.

I have revealed it and stored it in a note it should be fine then right?

Keys don't. Promotions do. If you claim a promotion, but don't actually reveal the key, it will still expire.

Oh i see, thanks everyone for the informative replies <3 :)

So if I reveal a promotional code (free games), will I be able to use it or give it to a friend after expiration date (after it expires)?