When idling in Team Fortress 2, do items build up?

According to this wiki items are dropped between 30 and 70 minutes. There is a weekly cap of 10 hours, which gets reset on Thursdays as 00:00 GMT.

If there it time left over after the reset, leftover time will be added to the next week. This is also capped, though. Leftover time can only max the current week to 20 hours.

As a side note, 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.

Also relevant information from here:

the amount of items a player can earn in a week is estimated to be around 8-12 items

and

As of the July 11, 2013 patch, the above method does not work anymore. Idling in textmode will not get you items. You must click on the item notifications as they come up or you will only get one item.