Idling in Team Fortress 2

You will not be punished for joining a server running "achievement_idle" and leaving your computer on.

However, the marginal gain idling provides means that it's not a very effective way of getting items, especially if running your computer for that long means overheating.

Valve never "punished" idlers, only those who used the external program developed by Drunken_f00l to trick the steam servers into thinking they were in the game, connected to the server, when in fact, they were only connected to the server, outside the game.

For what it's worth, I believe Valve stated that ~1 hour a night was enough to get all your weekly drops.

As Raven Dreamer said:

• look for servers with maps containing "idle";
• there is no punishment for idling (currently).

But I would like to complement on what you are trying to do.

The official Wiki states (https://wiki.teamfortress.com/wiki/Item_drop_system) :

Players are guaranteed to find items at regular intervals of 30 to 70 minutes, with an average interval of 50 minutes.

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.

So, your calculations should be: 10 items per week, 5 to 10 hours of gameplay.