manage internet data use in c#

There's some sample code with the documentation http://msdn.microsoft.com/en-us/library/system.timers.timer.aspx. As suggested by ThePhantomUpvoter below if you're using or writing a proxy server then try searching in codeproject for some further examples

I thought you wanted it based on duration not data use. Have a look here http://stackoverflow.com/questions/4106542/can-i-write-a-program-to-limit-the-internet-usage-to-40mb-per-day

It called a proxy server and there are many existing solutions around.