The TF2 system works like this
so I imagine it's pretty much the same thing (with the only difference being that they give out items at the end of the map instead of right away)
The current item drop system was introduced on April 20, 2010.[1] Players are guaranteed to find items at regular intervals of 30 to 70 minutes, with an average interval of 50 minutes.
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[2]. 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.[3][4] The system resets each player's playtime cap every Thursday at approximately 00:00 Greenwich Mean Time (GMT)[7:00 PM EST, 8:00pm EDT Wednesday]. 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).
so I imagine it's pretty much the same thing (with the only difference being that they give out items at the end of the map instead of right away)