[Weaseltail]

Here's what I can tell you from my experience . . . .

I have played six Challenge seasons and received four packs so far.

The packs have been awarded for different achievements, some mid-season and others during/after the playoffs.

I sim all the games and that hasn't seemed to impact receiving packs.

The pack awards appear to be random and not tied to any particular type of achievement (although the achievement level does seem to matter, see below).

I believe all the packs were awarded after receiving silver level or better achievements, which is consistent with what the developers have said in the past (higher % chance to get packs with silver/gold achievements than with bronze).

I don't know how long the cool down period is, but I believe it is at most two days. It may be shorter. It could also be tied to something other than real time (like in-game time, number or achievements, etc.).

You can earn packs for each of your teams if you have more than one, and they appear to be on separate cool down timers. For example, last night I received a pack when I was logged in to a Challenge game with one of my accounts/teams (I have 3 total in PT, which I believe is the max allowed). I then logged into another team, started a new Challenge game, and received another pack for the second team.

When you are awarded a pack, an in-game pop up appears. You can't miss it.