Not every game drops cards
Steam trading cards only exist for games that have opted into the Steam Trading Card system. On a store page you can check the feature list on the right-hand side: if "Steam Trading Cards" is not listed there, the game will never drop a card no matter how long you play it. Older indie titles, most demos and a large share of very cheap bundle games have no card support at all.
Free-to-play games are a special case. They do have cards, but the drops are not earned by playing. Steam grants them in proportion to money spent inside the game, so idling a free-to-play title produces nothing. If your goal is cards from playtime, only paid games in your library count.
Each game has a fixed allowance
A game does not drop cards forever. When you buy it you receive an allowance of drops that covers roughly half of the badge set, and once that allowance is used the game stops dropping cards permanently for your account. That is deliberate: Valve designed the system so that the remaining half has to be traded for, bought on the Community Market, or received from friends. It is what makes the card economy work.
You can see exactly what is left. Open your Steam profile, go to Badges, and each game shows how many card drops remain. Anything showing zero is finished. When people talk about "farming a library", they mean clearing those remaining counts across every game they own.
How drops arrive while you play
Cards are released for playtime, not for progress. Achievements, level-ups and in-game events make no difference; the only thing that matters is that Steam sees the game as running. Drops arrive at loose intervals, and about half an hour of playtime per card is a fair planning average. It is not a strict timer, so expect quiet stretches followed by two cards in quick succession.
Because it is playtime-based rather than input-based, leaving a game running while you do something else works the same as actively playing it. That is the entire principle behind idling, and it is also why the drop system feels invisible: you get a small notification in the corner and the card lands in your inventory.
Why a new account gets nothing
Fresh accounts almost always hit the same wall. Steam holds back the first card until a game has built up meaningful playtime — around three hours is the usual figure — and accounts that have never spent money are "limited", which restricts other community features as well. Someone who launches ten games for twenty minutes each and sees no cards has not broken anything; they simply have not cleared the warm-up in any of them.
Trading and the Community Market add their own gate. Both require Steam Guard running on the mobile authenticator, and there is a waiting period after you enable it before trades and market listings unlock. If you plan to buy the missing half of your sets, set that up early so the clock is already running.
From cards to badges to levels
One complete set — one of each card in that game, duplicates do not count — can be crafted into a badge. Crafting consumes the cards and returns 100 XP toward your Steam level, a random emoticon, a random profile background and a discount coupon for a game in the store. Foil cards drop far more rarely and craft into a separate foil badge worth another 100 XP, so a game you have already badged can still be worth more XP later.
Duplicates have no crafting value, which is why most collectors sell them on the Community Market and use the wallet balance to complete other sets. For older games with large player counts, buying a whole set outright is often cheaper than the time it would take to farm it, because supply is high and prices settle near the market minimum.
A practical order of operations
Start by opening your badges page and writing down which games still have drops remaining. Idle those first, since they cost nothing but time. Sell the duplicates you end up with, then use the balance to buy the missing cards for the sets you care about most. Craft, collect the XP, and repeat.
- Check the badges page for remaining drops before idling anything.
- Ignore free-to-play titles — playtime does not generate drops there.
- On a fresh account, budget about three hours per game before the first card.
- Sell duplicates rather than hoarding them; they never help a badge.
- Compare the market price of a full set against the hours idling would cost.