Internet time limit app

I can't recommend any specific ones since I haven't tried them. But if you search keywords Windows Parental Controls you should get hits. There's a parental control built into Windows Seven:
Parental Controls - Windows 7 features - Microsoft Windows

Also this page has reviews of some free ones:

Best Free Parental Filter

He's talking about how computers are controlled in cybercafes.

There is this "master" computer which the receptionist is sitting at, and a bunch of "user" computers (for customers). The user computers are normally locked - kinda like screensaver and no user input allowed. The only way to use them is to activate them via the master computer. So when you go to the cybercafe and tell the receptionist you want to use a PC, she uses the master pc to unlock one of the user PCs for your use. When your time is up, you go to pay and leave, and the receptionist re-locks the computer.

In some cybercafes they allow you to pre-pay a set period of usage, say for example 2 hours. So after 2 hours the user's computer will go back into locked mode. This way you don't need to wait for the receptionist to lock/unlock a pc.

This doesn't sound like what you're looking for, however. From your question I presume you just want to limit online connectivity while not actually restricting the use of your computer otherwise.

I'm not quite sure what your question is, now that you mention 15 PCs. Can you explain your setup and what you are trying to do?