Time limit on Linux?

A few months ago I installed Lubuntu on my kids laptop, but going from windows to Linux has its challenges. 

One thing that I found really easy to set up in windows is a time limit for user accounts, so my kid wouldn't sit by the computer all day after school and all weekends. Does anyone know how to setup a similar "time limit" for her account? 

And if anyone have additional suggestions for parental control when using linux feel free to add them here. 

Tnx / tInc 

Maybe have a look at this; Linky

Or if you need ideas on a cron job based script; Linky

Timekpr looks really nice! Will try ASAP. Thanks!

so my kid wouldn't sit by the computer all day after school and all weekends

Stop being evil.

Its important for kids to get out at a young age and learn how to make friends and socialize. I know this is a nerdy site but I certainly dont want my kids being some geek too scared to talk to a girl.