How often do Booster Packs drop? [duplicate]

I know how Booster Packs drop, but I am wondering just how often they drop. Is it random? Do they drop when you get achievements in games? Do they drop when you buy a game?

As per the official Steam Community page on Trading Cards, "Booster packs are granted randomly to eligible users as more badges are crafted by members of the community." (Eligible meaning "people who have received all their possible card drops for that game".)

So, they are not from achievements or game purchases, they are somewhat random, and there is no way to know for certain how often they will drop, since that will vary as more/less people craft badges.

The only real way you can directly influence their rate is to level up: every ten Steam levels you gain will increase your chances of getting a booster pack by 20% of their normal chance; get to something really high like level 50, and you have twice the chance you otherwise would.