This may be really controversial so feel free to tell me to bogg off and mind my own business - especially as I dont have kids and nor do I want any (used to be a teacher but they were only 6 yrs old!!)
Why do you want to restrict her time on-line? Is it due to cost or something else? If its cost then fine put a restriction on it. If its that you think she ought to be doing something else with the time she spends on line then I'm not so sure.
There are loads of things I really ought to be doing right now but instead I'm 'wasting' time on here. I could be doing housework, catching up with the pile of work on my desk, cleaning the car, going to Tesco's - any number of things but I'm not. Is it any different for teenagers.
My only proviso would be that homework HAS to be done before time is spent on MSN, but apart from that there are much worse things she could be doing than spending time talking complete cobblers with her mates
JMO - no offence intended