In tf2, the drop system has had some restrictions put in place to stop users from idling. These restrictions include:
-User must be in a VAC secured server
-User must be responding to in-game drop notifications while not utilizing external commands to do so
-Is running one instance of the game
-Is not in text mode

Some other things to take note about the drop system:
-The drop system currently drops items at regular intervals of 30 to 70 minutes. However this has a cap of 10 hours per week, play beyond the cap and you won;t find anymore items.
-Full backpacks shouldn't affect the item drop system, it should just prompt you to either delete an item in your inventory to make room, or just delete the item you find.

source: https://wiki.teamfortress.com/wiki/Item_drop_system

Thank you but how do i effectivly get amount items as little time as possible then??

Play the game 10 hours each week, it wont get faster than that.