Assuming you are farming efficiently...

(leaving when you are confident it will be gold, prioritizing 2 that are near each other over others, prioritizing easy to win fates if alone, etc)

Assuming all of this, your droprate per atma will likely range between zero and 10 hours each.

Anything beyond that is extraordinarily rare. So rare, I have never seen it, and I have over 60 atma farmed at this point.