It's now 6 months and 26 days after I had 1000 free Supercharger miles added to my account from a referral delivery, and I still have some of these miles remaining on my account today. And I just Supercharged 2 days ago and didn't get billed -- just deducted from my remaining free miles balance.
I wonder if this is just a lucky error on my account, or if this whole "doesn't-actually-expire-after-6-months" thing is more widespread.
Can anyone else add some more data points to this picture?