I think this should be do-able, although I think its difficult to define the time-spent on the internet.
From what I know, e.g. Bluecoat proxies do not have this feature.

From the other hand you could script it maybe. Let someone login before they can surf over a proxy and setup a timer. Let the timer add up, every time someone logs in ...

Then again, It is difficult to see if someone is actually using the internet, even if you have an open session over a proxy. It can be idle.