It varies from game-to-game. A rule of thumb (the most common time) is usually 2 hours for the first card, then every 40 minutes thereafter until done.

That said don't be discouraged if 2 hours rolls around with no change or freak out if a card arrives early. There's variation in time, so be sure to take the time-frame with a pinch of salt. As two personal examples, I spent 9 hours getting 3 cards from "Holy potatoes a weapon shop" and have spent only 84 minutes to get all 3 Hotline Miami cards.

Got 10 hours on Reus and no cards yet, xD.

Freaking Reus. There's one horrible game to idle.