This Christmas, like always, my cousins will be coming over for a visit and undoubtedly they will want to play on my computer. The problem is, it causes havoc every time. Not only are they really hard to force off of my computer but they (two brothers) get real upset if one gets slightly more time than the other.

So this year I'm hoping I'll be able to setup my computer so that who ever logs into the guest account gets kicked off in around half an hour whether they are in the middle of playing doom 3 or not.

The login part is no problem, I can just set the password myself and log them on when it's their turn. But I'm not really sure how to kick them off after half an hour, I was thinking something like:
But I don't know where i could put that.

It doesn't have to be terribly secure, they're 11 and 12 years old and have no linux experience.

I'm using up-to-date x86 gentoo linux.

I haven't used it, but sounds like cron may be of some help. emerge the vixie-cron and see if it can work for you

Nope cron wont do. I want them to be logged off half an hour from logging on. If i used cron then if they logged in at 5:15pm then they would be logged off at 5:30pm, giving them only quarter of an hour as opposed to the full half hour