Do you know of any application that will lock my Mac for a few hours?

It’s called the power button. Press it, leave the room, find three strong friends and shove your piano against the outside of the door, then go outside and play.

Power it down and lock the CPU section in a safe with a time controlled lock.

Any software solution can be circumvented. Therefore, the answer is no.

Personally, I have a wife that reminds me what time it is and when I need to get up in the morning; far more effective and harder to get around.

There’s an app called Concentrate that lets you block certain apps during activity periods you create. Perhaps you could make one that basically blocks out everything and call it sleep or something.

It’s not nearly as bulletproof as @jerv‘s solution though. :)

You could use Applescript to give you a warning and shutdown.

http://i.imgur.com/PhuAw.png

And then use iCal to run the script every thirty minutes or so. It won’t force you out but it will annoy the shit out of you. Just save the script above and call it as an alarm as often as you like. You can remove the first line of the script and it will just Shutdown without a warning.

http://i.imgur.com/P1Aw7.png

Does OS X have an equivalent to the Windows ‘times’ switch?

In Windows XP, I can enter this at the command line to limit my allowed login times:
“net user jaytkay /times:M-F,8am-8pm; Sa,8am-5pm;Su,8am-1pm”

Windows 7 and Vista have a graphic interface to handle it, ‘Parental Controls’

@jaytkay You would think so, considering that OS X is basically BSD and there are Linux programs for nearly everything else. As much as Apple loves to remove options in the name os “ease of use”, somehow I doubt that they went that far, so worst case scenario is a little terminal time… though I would bet that somebody out there has written it already and we just don’t know about it.