How Steam card drops are earned
Steam gives out trading cards for playtime, not for achievements or progress. Launch a game that supports Steam Trading Cards and, as long as you still have drops remaining for it, cards arrive at loose intervals while the game is marked as running. Around thirty minutes per drop is the figure most collectors plan around. It is an average rather than a rule: drops are released in bursts, so a long session can be quiet and then produce two cards a few minutes apart.
The important limit is that each game has a fixed allowance. Steam grants roughly half a set through playtime and then stops, permanently, for that game. Once you have used the allowance, no amount of extra idling produces another card — the remaining cards have to be traded for or bought on the Community Market. That is why the calculator asks for drops remaining rather than set size. You can see the remaining count for any game in your badge page on the Steam community site.
Fresh and limited accounts
A Steam account that has never spent money is “limited”, and new accounts are also treated cautiously by the drop system. In practice a game needs to accumulate about three hours of playtime before the first card is released. This is the single biggest reason people think idling is broken: they idle ten games for twenty minutes each, see nothing, and assume it failed. Tick the limited-account box above and the calculator adds that warm-up to every game so the estimate matches reality.
Two other restrictions matter on fresh accounts. Trading and Community Market access require Steam Guard on the mobile authenticator, and there is a waiting period after enabling it before you can trade. And free-to-play games do not drop cards for playtime at all, so they contribute nothing no matter how long they idle.
Making the calendar time smaller
The total number of idle hours is fixed by how many drops you have left. What you control is how many of those hours run at the same time. Idling several games in parallel divides the wall-clock time without changing the total, which is why the calculator has a “games idled at once” field. Running continuously matters too: a session that only runs while your PC is on takes far longer in calendar days than the 24/7 figure shown above.
Once the cards are in, turn them into levels — the Steam level calculator shows how many badges each level costs, and the guide on how to get Steam trading cards covers which games qualify.