Time limits on MSN

I wanted to find out if anyone knows of a way to limit the time spent using
MSN. I have bought my teenage daughter a laptop which she now uses in her
bedroom.
I want to give her as much freedom and trust as I can but time ebbs away
when chatting to her friends. I have installed Symantec Norton 360 to help
with this but neither 360 nor Vista allows me to limit the time spent using
MSN. I have used Vista Parental Controls to allow a sensible time limit but
this gives 5 hours of access when I would like to restrict MSN time to 2
hours per day.

Any suugestions gratefully received, even if that requires to purchase some
additional software.
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Best regards
Paul