If you were offered free unlimited supercharging for six months your account would show an expiration date.
If you were offered a fixed amount of supercharging miles (1,000 or 5,000 baseD on recent bonus programs) those miles only show up in your loot box on the phone app and the expiration date will not show up. We don’t really know how closely Tesla is tracking those expiration dates since there haven’t been a lot of posts from people who noticed them having expired.
The rules also state that for each additional referral code someone uses, you get an additional set of bonus miles AND your existing miles are extended for six additional months. But again none of this is displayed in the loot box so you won’t see an expiration date there.