That Chinese law is now in effect and games that feature loot boxes (or in this case, card packs) must now reveal their odds.
League of Legends was one of the first ones out the gate, but now Blizzard has revealed the odds of getting the different card rarities out of packs. On average, players will find a legendary 1 out of every 20 packs and an epic card 1 out of every 5 packs. Players have also surmised that Hearthstone’s “back luck protection” kicks in at about 40 packs. So if somehow you get unlucky enough to go 40 packs without a legendary, that’s about when it kicks in and gives you one for free.
Hearthhead has all the data, and interestingly, though Blizzard didn’t reveal the odds of getting a golden card, they have some data on that as well thanks to a community effort. About 1 out of every 17 packs gets you a common, 1 out of every 20 gives you a golden rare, 1 out of 100 packs for a golden epic, and unfortunately, there isn’t enough data to really know about golden legendaries. Suffice it to say, they’re rare.
Of course, while this doesn’t necessarily mean the chances are the same across all regions, so far it seems like they’re at least roughly similar.