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Refer to Google, it's too extensive to be covered here.

引用自 .hazu：

and here i am idling for 7,3 hours and only got 2 card drop ='D

I noticed similar behaviour with very few games, I had that impression for a while that some games require a lot more time to drop all cards.

引用自 Glimmer：

This is no longer 100% true. Some accounts are getting drops prior to the 2 hour mark [...] seems to be tied to whether the account has requested a refund [...] contrary reports so it's not certain what governs the requirement.

Thanks for the information, it sure is interesting why it happens. My previous answer was based entirely on the author's information, as well as on my own experience. I haven't gotten a single card before 2 hours mark, but on numerous occasions I got a card within 2 minutes after 2 hours mark.

The author once explained that you can safely idle multiple games for up to 2 hours each, as cards don't drop for games you haven't played for 2 hours yet.
After you've played/idled a game for 2 hours you become eligible to get cards, then however, you have best chances to get cards if you play/idle single game, if you play multiple games the chance to drop a card is reduced.
Note, this also applies if you play/idle a game with over 2 hours play time and idle at least one other game at the same time.