If you want the skinny on the drop system...
"The drop system distributes a random items to players on a regular basis until a weekly time-cap is reached.
Players are guaranteed to find items at regular intervals of 30 to 70 minutes, with an average interval of 50 minutes.
It has been estimated that a maximum of 5 to 12 items can be found each week. Also the system has a cap on the amount of playtime in which drops can occur. This cap has been estimated to be 10 hours each week. Playing beyond the cap will not yield additional items.
Mann Co. Supply Crates and other usable items drop on a separate timer, meaning that these items do not affect the interval between regular drops.
The system resets each player's playtime cap every Thursday at approximately 00:00 Greenwich Mean Time (GMT)[7:00 PM EST]. If a player plays less than the playtime cap in a week, their leftover time will be added to the next week, so a player who does not play for one week will receive twice as many items the following week. The amount of time a player can save up is capped at double the allotment of a single week (roughly 20 hours).
The dropped items are stored in the Backpack after the player's death, after a trade is completed, if the Mann Co. Store is opened, or if ten duels are won. If the player leaves the server after receiving an item, but before storing it in the backpack, a notification will alert the player of the new item(s) when the Main Menu is visited the next time. Occasionally, dropped, traded, or crafted items may get stuck "in limbo" and may not immediately appear in the player's Backpack. This may occur on instant respawn servers blocking the "New Item Found!" screen pop-up. Visiting the Mann. Co. Store or disconnecting from the server will usually restore these items to the player's Backpack.
While it has always been perceived that each weapon has an equal chance of being dropped to the player, it appears that certain weapons that form part of an item set will drop more or less often than others. It is not known exactly which of these weapons are rarer or more common.
Under the Free-To-Play system introduced in the Über Update, free players receive only limited item drops (with no rare or cosmetic items) until they upgrade to Premium status, upon which they gain access to the full item drop system."
TL;DR?
> Time cap for drops
> Guaranteed to get drops
> Not playing = more drops later
> Hats + more valuable weapons drop far less often
> Some items do not drop at all
> Crates and other tools (Paint, Decals, etc.) drop on a different timer
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