What timer?
Here's a Reddit post discussing it.
https://www.google.com/amp/s/amp.re..._opening_and_the_best_strategy/?client=safari
Basically every time you open a pack and don't get a legendary your odds of getting a legendary in your next pack increase. This keeps going until you have opened 39 packs without a legendary and there's a 100% chance your next pack will have a legendary.
The timer is separate for each set of cards. For example you can open up 39 basic packs without getting a legendary and your next basic pack will have a guaranteed legendary but your next karazhan or mean streets pack won't because they are on separate timers.
It's why you won't see anyone opening 100 packs of the new expansion and not getting a single legendary. Without this timer the odds of doing it would be low, but with enough people opening over 100 packs you'd be bound to have at least one really unlucky person who didn't get any legendaries.