Pity Timer on Packs Opening, and the Best Strategy: https://www.reddit.com/r/hearthstone/comments/3z7jyh/pity_timer_on_packs_opening_and_the_best_strategy/
According to the findings, it's a nearly guaranteed legendary card at 40 packs so it is similar to getting one of the highest rarity card when you open one box of TCG booster packs in real life.
